Opportunity and volunteer sampling | AQA A-Level Psychology Revision

For 7182 specification, first teach in September 2025

AQA A-Level Psychology | Free Revision Notes

Estimated study time: 45 minutes

These Opportunity and volunteer sampling A-Level Psychology revision notes explain two practical methods of recruiting research participants. You will learn how researchers select people who are readily available or invite people to put themselves forward. You will also examine opportunity bias, volunteer bias and the effects of recruitment location, timing and advertising. This lesson builds on the distinction between a target population and a sample and contrasts convenient recruitment with more structured sampling procedures.

Learning Objectives 🎯

By the end of this revision page, you should be able to:

• Define opportunity sampling and volunteer sampling.

• Explain how each sampling method is carried out.

• Distinguish researcher-selected opportunity samples from self-selected volunteer samples.

• Identify possible sources of sampling bias in unfamiliar scenarios.

• Evaluate the practical strengths and representativeness of both methods.

• Explain how each method affects generalisation.

Revision Notes 📚

Opportunity and volunteer sampling A-Level Psychology revision overview

Sampling is the process used to select participants from a target population.

Opportunity and volunteer sampling are commonly used because they are practical and do not require a complete list of the population.

They differ in who makes the initial decision about participation:

Opportunity sampling	Volunteer sampling
The researcher approaches available people	People respond to an invitation
The researcher selects from those present	Participants select themselves
Recruitment occurs at a convenient place and time	Recruitment occurs through an advertisement or appeal
Main risk is availability and researcher-selection bias	Main risk is volunteer or self-selection bias

Both methods may produce samples that differ systematically from the target population.

This can reduce:

• Representativeness.

• Generalisability.

• Confidence that the findings apply beyond the participants tested.

Specification boundary

The AQA specification requires knowledge of:

• Opportunity sampling.

• Volunteer sampling.

• The implications of sampling methods.

• Bias.

• Generalisation.

The specification does not prescribe one compulsory recruitment procedure for either method.

Researchers must explain exactly how participants were selected in the investigation being described.

Opportunity sampling

What is opportunity sampling?

Opportunity sampling involves selecting participants who are available and willing to take part at the time the researcher needs them.

It is sometimes described as selecting people who are:

• Convenient.

• Accessible.

• Nearby.

• Available at a particular time.

• Easy for the researcher to approach.

For example, a psychologist may recruit:

• Students in a college common room.

• Customers leaving a shop.

• People using a library.

• Employees working in one office.

• Members of a class available during a free period.

• People passing a particular location.

The researcher does not normally begin with a complete sampling frame.

Example of opportunity sampling

A psychologist wants to investigate revision habits among sixth-form students.

She approaches the first 30 students who are available in the college library during lunchtime and invites them to complete a questionnaire.

This is opportunity sampling because:

• The students are selected because they are available.

• Recruitment takes place at a convenient location and time.

• The researcher approaches the participants directly.

• Other students in the target population do not have an equal chance of selection.

Opportunity does not mean everyone must agree

People approached by the researcher still have the right to refuse.

The opportunity sample consists of those who are:

1. Available.

2. Approached or selected.

3. Eligible.

4. Willing to participate.

A person does not lose the right to informed consent simply because they were conveniently available.

How opportunity sampling is conducted

A researcher using opportunity sampling may:

1. Define the target population.

2. Choose a convenient recruitment location.

3. Choose a recruitment time.

4. Identify people who meet the eligibility criteria.

5. Approach available people.

6. provide information about the study.

7. Obtain informed consent.

8. Continue until the required sample size is reached.

The procedure should be described clearly.

Defining the target population

The researcher should begin by identifying the group they want to understand.

For example:

All Year 12 and Year 13 students attending Westford College.

The opportunity sample might then consist of:

Forty students available in the college café at lunchtime.

The sample belongs to the population, but it may not represent the full population accurately.

Choosing a location

The recruitment location can influence who is available.

For example, recruiting in a college library may overrepresent students who:

• Use the library.

• Remain on site during free periods.

• Complete independent study.

• Have no lesson at that time.

• Are comfortable working in a quiet environment.

It may underrepresent students who:

• Study at home.

• Spend free periods elsewhere.

• Have work or caring responsibilities.

• Are absent.

• Avoid the library.

The location can therefore create sampling bias.

Choosing a time

The time of recruitment can also affect who is included.

For example, recruiting students:

• Before lessons.

• During lunch.

• After school.

• During examination season.

• On one particular weekday.

may produce different samples.

Students available after school may differ from students who leave immediately because of:

• Transport.

• Employment.

• Family responsibilities.

• Extracurricular activities.

• Motivation to remain at college.

Researcher selection

The researcher may choose which available people to approach.

This creates an opportunity for researcher selection bias.

A researcher might approach people who:

• Appear friendly.

• Seem likely to agree.

• Are not visibly busy.

• Are similar in age to the researcher.

• Appear confident.

• Are personally familiar.

• Seem interested in the topic.

People who appear less cooperative or harder to approach may be excluded.

Making opportunity selection more systematic

Opportunity sampling cannot give every population member an equal chance of selection, but the researcher can reduce personal choice.

For example, they could approach:

• Every person entering the location during a defined period.

• Every third eligible person who passes a particular point.

• Everyone seated in a particular section.

• All eligible people present at the beginning of a session.

This creates a clearer procedure.

However, it remains an opportunity sample because only people available in that setting and period can be selected.

Opportunity sampling and a sampling frame

Opportunity sampling does not usually require a full list of the target population.

This makes it useful when:

• A complete list is unavailable.

• Recruitment must happen quickly.

• The study is small.

• The population is difficult to identify fully.

• The researcher has access to a particular setting.

The lack of a sampling frame also makes it difficult to know whether the sample reflects the target population.

Strengths of opportunity sampling

Strength: quick recruitment

Opportunity sampling can be carried out rapidly.

The researcher approaches people who are already:

• Present.

• Accessible.

• Near the research setting.

• Available to begin the procedure.

This may allow data collection to begin immediately.

For example, a researcher carrying out a classroom study may recruit students from an available class rather than contacting people across the whole college.

Strength: convenient

The method is convenient because the researcher does not need to:

• Obtain a complete population list.

• Assign identification numbers.

• Generate random selections.

• Calculate sampling intervals.

• Contact people across several locations.

This may be especially useful for a small investigation with limited resources.

Strength: inexpensive

Opportunity sampling can reduce costs involving:

• Travel.

• Advertising.

• Administrative work.

• Accessing population records.

• Contacting selected people individually.

• Arranging multiple recruitment sessions.

The researcher may recruit participants in a setting they already use.

Strength: practical when no sampling frame exists

Some target populations cannot be identified through a complete list.

For example, a researcher may want to study:

• People using a public space.

• Customers visiting a particular location.

• People attending an event.

• Members of a changing online community.

Opportunity sampling allows the researcher to recruit from those who are accessible.

Strength: useful for preliminary research

A researcher may use an opportunity sample to:

• Test a procedure.

• Check instructions.

• Estimate how long a task takes.

• Identify practical problems.

• Explore whether a research question is workable.

This may be helpful when testing research procedures before the main investigation.

However, findings from a convenient preliminary sample should not be treated as representative without further evidence.

Strength: participants can be screened directly

The researcher can check immediately whether an available person meets the study’s inclusion criteria.

For example, they may confirm:

• Age.

• Course studied.

• Relevant experience.

• Ability to complete the procedure.

• Previous participation in a similar task.

This can prevent ineligible people from entering the study.

Limitations of opportunity sampling

Limitation: sampling bias

Opportunity sampling is likely to produce bias because participants are selected from a limited location and time.

People who are available may differ systematically from those who are not.

For example, students present in a library may be more academically motivated than the wider student population.

If motivation affects the measured behaviour, the findings may be distorted.

Limitation: poor representativeness

A convenient sample may fail to reflect relevant characteristics of the target population.

It may overrepresent people who:

• Use one setting.

• Attend at one time.

• Are easy to approach.

• Are comfortable speaking to researchers.

• Have flexible schedules.

• Live or work near the research site.

This reduces confidence that the sample represents the population.

Limitation: researcher bias

The researcher may consciously or unconsciously select particular people.

For example, they may approach participants who:

• Look cooperative.

• Appear to have time.

• Share similar characteristics.

• Seem likely to understand the task.

• Are personally known.

The sample may therefore reflect the researcher’s choices rather than the target population.

Limitation: one setting may be unusual

People recruited from one setting share environmental influences.

For example, students from one school may experience the same:

• Teaching.

• Rules.

• Timetable.

• Resources.

• Local culture.

• Examination preparation.

Findings may not generalise to students in other settings.

Limitation: time-related bias

Recruitment at one time may exclude people with different schedules.

For example, approaching shoppers on a weekday morning may overrepresent:

• People who do not work standard daytime hours.

• Retired people.

• People with flexible schedules.

• People available in that area at that time.

The sample may not represent all customers.

Limitation: people may feel pressured

People approached directly may feel uncomfortable refusing, particularly when the researcher is:

• Their teacher.

• Their manager.

• A person in authority.

• Someone they know.

Researchers must make clear that participation is voluntary and that refusal carries no negative consequences.

Limitation: generalisation may be weak

The findings may apply accurately to the people tested but not to the wider population.

For example:

Thirty students recruited from a Psychology classroom preferred one type of revision activity.

This does not automatically show that:

• All students share the preference.

• Students studying other subjects agree.

• Students at other colleges agree.

• Young people outside education agree.

The conclusion should remain appropriately limited.

Volunteer sampling

What is volunteer sampling?

Volunteer sampling involves asking people to put themselves forward to participate in research.

It is also called self-selected sampling because potential participants decide whether to respond to the invitation.

Researchers may recruit volunteers through:

• Advertisements.

• Posters.

• Emails.

• Social-media posts.

• Websites.

• Newsletters.

• Notices within an organisation.

• Appeals made during a lesson or meeting.

The people who respond form the volunteer pool.

Example of volunteer sampling

A psychologist posts an online advertisement asking sixth-form students to participate in research about examination anxiety.

Students who complete the response form put themselves forward.

This is volunteer sampling because:

• Participation begins with a general appeal.

• Individuals decide whether to respond.

• The researcher does not select directly from the entire population.

• People interested or willing to take part are more likely to enter the sample.

How volunteer sampling is conducted

A researcher may:

1. Define the target population.

2. Prepare an advertisement or appeal.

3. State the eligibility requirements.

4. Distribute the appeal through selected channels.

5. Receive responses from potential participants.

6. Check that volunteers meet the inclusion criteria.

7. Provide full information and obtain consent.

8. Recruit the required number.

The advertisement

A volunteer advertisement may include:

• The general topic of the research.

• Who is eligible.

• What participation involves.

• Approximate duration.

• Location.

• Researcher contact details.

• Any compensation or reward.

• How to express interest.

The advertisement should provide enough information for people to decide whether to make contact without using pressure or misleading claims.

Advertisement placement

Where an advertisement is displayed affects who sees it.

For example, an online advertisement may exclude people who:

• Do not use that platform.

• Have limited internet access.

• Do not follow the relevant account.

• Do not see the post because of how content is displayed.

• Prefer not to respond online.

A poster in a university Psychology department may overrepresent:

• Psychology students.

• People interested in research.

• People familiar with psychological terminology.

Self-selection

The defining feature is that participants select themselves.

People who volunteer may be more:

• Interested in the research topic.

• Curious about psychology.

• Confident.

• Available.

• Motivated by a reward.

• Comfortable sharing personal information.

• Eager to help researchers.

People who do not volunteer may be:

• Busy.

• Uninterested.

• Concerned about privacy.

• Uncomfortable with the topic.

• Unable to attend.

• Unaware of the advertisement.

These differences can create volunteer bias.

Volunteer pool and final sample

Sometimes more people volunteer than the researcher needs.

The researcher might then:

• Recruit eligible volunteers in order of response.

• Randomly select from the eligible volunteers.

• Select volunteers who meet particular criteria.

Randomly selecting from the volunteer pool does not turn the sample into a random sample of the target population.

Every member of the target population did not have an equal chance of entering the volunteer pool.

The sample remains volunteer-based.

Volunteer sampling and informed consent

Responding to an advertisement does not complete the consent process.

The researcher must still explain:

• The procedure.

• Possible risks.

• Data use.

• Confidentiality.

• Withdrawal rights.

• Recording.

• Debriefing.

Expressing interest is not the same as providing fully informed consent.

Strengths of volunteer sampling

Strength: convenient recruitment

Volunteer sampling allows researchers to reach potential participants without approaching every person individually.

One advertisement may be distributed to:

• An entire college.

• A large organisation.

• A wide online audience.

• Members of several groups.

This can make recruitment relatively straightforward.

Strength: can produce a large response

An advertisement distributed widely may generate many potential participants.

This gives the researcher an opportunity to:

• Screen for eligibility.

• Recruit the required sample.

• Select participants meeting particular criteria.

• Maintain a reserve list in case of withdrawal.

A large response does not guarantee a representative sample because all respondents remain self-selected.

Strength: useful for dispersed populations

Advertisements can reach people across:

• Different locations.

• Multiple institutions.

• Wide geographical areas.

• Online communities.

This may be more practical than recruiting only from one physical setting.

However, the advertisement channels may still exclude sections of the population.

Strength: participants are willing to take part

Volunteers have made an active decision to respond.

They may therefore be:

• Interested in participating.

• Prepared to attend.

• Willing to follow instructions.

• Motivated to complete the procedure.

This may reduce difficulties associated with recruiting people who do not wish to participate.

However, high motivation may also make volunteers different from the target population.

Strength: useful for demanding studies

Volunteer sampling may be practical when research requires participants to:

• Attend several sessions.

• Complete a lengthy interview.

• Keep a diary.

• Travel to a laboratory.

• Discuss a particular experience.

• Complete repeated measures.

People who have actively volunteered may be more prepared for the commitment.

Strength: may help reach people with particular experiences

A targeted advertisement can request participants with a specific characteristic or experience.

For example:

Students who returned to education after a break of at least three years.

This may help researchers locate suitable participants who are otherwise difficult to identify.

The resulting sample may still be unrepresentative of everyone with that experience.

Limitations of volunteer sampling

Limitation: volunteer bias

Volunteer bias occurs when people who volunteer differ systematically from those who do not.

Volunteers may be more:

• Interested in psychology.

• Motivated.

• Confident.

• Comfortable expressing opinions.

• Available.

• Attracted by the topic or reward.

Their behaviour or responses may not represent the target population.

Limitation: topic interest

People with a strong interest in the research topic may be particularly likely to respond.

For example, an advertisement about examination anxiety may attract students who:

• Experience high anxiety.

• Have strong opinions about examinations.

• Have previously received support.

• Believe the topic is personally important.

Students with little anxiety may ignore the advertisement.

The sample could therefore overestimate anxiety in the wider population.

The opposite pattern is also possible. Highly anxious students may avoid taking part because the topic is uncomfortable.

Limitation: stronger opinions may be overrepresented

People with particularly positive or negative views may be more motivated to participate.

Suppose a researcher advertises a study about attitudes towards school rules.

Students with strong opinions may respond, while students with moderate or indifferent views may not.

The findings may exaggerate the strength of attitudes across the population.

Limitation: availability

Volunteers must have enough time and flexibility to participate.

People with:

• Employment.

• Caring responsibilities.

• Travel difficulties.

• Busy schedules.

• Limited access to the research setting.

may be underrepresented.

The sample may therefore contain people with unusually high availability.

Limitation: advertisement bias

The wording and placement of the advertisement may attract particular people.

For example, an advertisement stating:

Do you struggle with stressful examinations?

may attract students who identify with examination stress.

A more neutral wording might be:

Participants needed for research into experiences of examinations.

Researchers should avoid wording that reveals a desired response or appeals only to one part of the population.

Limitation: access to advertising channels

Not every population member has an equal opportunity to see the appeal.

An online advertisement may exclude:

• People without regular internet access.

• People who do not use the platform.

• People who use different languages.

• People whose accessibility needs are not met.

A poster in one workplace excludes employees based elsewhere.

Limitation: rewards may influence the sample

Offering payment, vouchers or another reward may increase recruitment.

However, the reward may be particularly attractive to people with certain circumstances.

Researchers should ensure that the incentive:

• Compensates reasonably for time or inconvenience.

• Is not so large that people feel unable to refuse.

• Is described accurately.

• Does not encourage unsuitable participants to hide relevant information.

Limitation: dishonest eligibility claims

Some volunteers may claim to meet eligibility requirements because they:

• Want the reward.

• Are interested in the procedure.

• Believe the requirement is unimportant.

• Misunderstand the criteria.

Researchers should check eligibility consistently and sensitively.

Limitation: generalisation

A large volunteer sample can still be unrepresentative.

For example:

Ten thousand people respond to an online wellbeing questionnaire.

The sample may still overrepresent people who:

• Use the website.

• Notice the advertisement.

• Have time to participate.

• Are interested in wellbeing.

• Feel comfortable completing online questionnaires.

Sample size does not remove self-selection bias.

Opportunity and volunteer sampling compared

Feature	Opportunity sampling	Volunteer sampling
Who initiates selection?	Researcher approaches available people	Potential participants respond to an appeal
Main source	Convenient place and time	Advertisement, poster, email or online appeal
Participant choice	Can accept or refuse after being approached	Chooses whether to put themselves forward
Sampling frame required?	Usually no	Usually no complete population frame
Main advantage	Fast and convenient	Can reach many interested participants
Main bias	Availability and researcher-selection bias	Volunteer or self-selection bias
Population coverage	Limited to those available	Limited to those who see and answer the appeal
Cost	Often inexpensive	May require advertising and screening
Generalisation	Often limited	Often limited

Direct distinction

A strong distinction might state:

In opportunity sampling, the researcher selects people who are available at a convenient place and time. In volunteer sampling, potential participants select themselves by responding to an advertisement or request.

Both involve willingness

It is incorrect to say that only volunteer participants choose whether to take part.

All participants should be able to give or refuse consent.

The difference is:

• In opportunity sampling, the researcher initially approaches or selects the person.

• In volunteer sampling, the person initially responds to a general invitation.

Both may be practical

Both methods avoid the need for:

• A complete sampling frame.

• Numbering every population member.

• Complex proportional calculations.

• Contacting randomly selected people.

This often makes them faster than chance-based and interval-based procedures.

Both may be biased

Opportunity sampling may overrepresent people who are:

• Present.

• Accessible.

• Easy to approach.

Volunteer sampling may overrepresent people who are:

• Interested.

• Motivated.

• Willing to respond.

Both can produce samples that differ systematically from the population.

Sources of bias in opportunity sampling

Location bias

The place of recruitment may attract particular people.

Example:

Recruiting in a gym to study exercise attitudes.

The sample is likely to overrepresent people who:

• Exercise.

• Have gym membership.

• Feel comfortable in that environment.

• Have time and resources to attend.

Time bias

The time of recruitment may exclude parts of the population.

Example:

Recruiting in a town centre at 10.00 am on a weekday.

The sample may underrepresent people working standard daytime hours.

Researcher-choice bias

The researcher may approach participants selectively.

Example:

Choosing people who appear friendly or unoccupied.

This may exclude:

• Shy people.

• Busy people.

• People who appear less confident.

• People from groups the researcher finds harder to approach.

Setting bias

A sample from one institution may share experiences that do not apply elsewhere.

Example:

Recruiting all participants from one highly selective school.

The findings may not represent students in other educational settings.

Availability bias

Some people are more available because of their lifestyle or circumstances.

The sample may therefore underrepresent people with:

• Employment.

• Caring responsibilities.

• Transport difficulties.

• Health limitations.

• Strict schedules.

Sources of bias in volunteer sampling

Self-selection bias

People choose whether to respond.

Those who volunteer may differ psychologically or practically from those who do not.

Interest bias

People interested in the topic are more likely to notice and answer the appeal.

Strong-opinion bias

People with especially positive or negative views may be more motivated to participate.

Advertising-channel bias

Only people who use or access the selected channel are likely to see the invitation.

Incentive bias

A reward may appeal more strongly to particular groups.

Eligibility and access bias

The recruitment process may be difficult for people who:

• Need accessible formats.

• Speak another language.

• Lack digital access.

• Cannot attend at the required time.

• Cannot travel to the research location.

Sampling bias and generalisation

What is sampling bias?

Sampling bias occurs when the sample differs systematically from the target population because of the way participants were selected.

The sample may:

• Overrepresent one group.

• Underrepresent another group.

• Exclude some population members.

• Contain people with unusually strong motivation or availability.

How bias affects findings

Suppose a researcher investigates revision habits using students attending an optional revision workshop.

The sample may contain students who:

• Revise more frequently.

• Are more concerned about achievement.

• Are more willing to seek support.

The researcher may overestimate how often the wider student population revises.

How bias affects generalisation

Generalisability concerns whether findings can be applied beyond the sample.

A biased sample limits generalisation because:

• The participants may not reflect the wider population.

• The measured behaviour may be connected with the reason they were selected.

• Groups absent from the sample may respond differently.

A researcher should state where findings can reasonably apply.

Generalising from an opportunity sample

Suppose 40 students are recruited from one college library.

The findings may apply most confidently to:

• The particular participants.

• Possibly similar students using that library.

They should not automatically be generalised to:

• Every student in the college.

• All A-Level students.

• All teenagers.

Generalising from a volunteer sample

Suppose 2,000 people respond to an online advertisement about sleep.

The findings may apply most confidently to:

• People who saw the online appeal.

• Eligible individuals willing to complete the study.

The sample may not represent all adults because people who volunteered could differ in:

• Sleep concerns.

• Internet use.

• Interest.

• Availability.

• Motivation.

Large samples and bias

A large sample can improve the stability of results but cannot automatically remove sampling bias.

For example:

• A survey receives 20,000 volunteer responses.

• Every respondent was recruited through the same specialist website.

The sample remains restricted to people who:

• Use the website.

• Saw the invitation.

• Chose to respond.

A smaller sample using proportionate representation of relevant population groups might support stronger generalisation.

Evaluating opportunity sampling

Practical judgement

Opportunity sampling is strongest when the researcher needs:

• Rapid recruitment.

• A small manageable sample.

• Participants from a particular setting.

• A practical method without a sampling frame.

• People available to complete the procedure immediately.

Methodological judgement

Its main weaknesses are:

• Researcher selection.

• Restricted location.

• Restricted recruitment time.

• Availability bias.

• Limited representativeness.

• Weak generalisation.

Balanced evaluation

Opportunity sampling is quick and inexpensive because the researcher recruits people who are already available. This can make a study practical when there is no complete sampling frame. However, those present in one location at one time may differ from the target population, and the researcher may choose people who appear cooperative. The resulting sample may therefore be biased and the findings may not generalise.

Evaluating volunteer sampling

Practical judgement

Volunteer sampling is strongest when:

• A wide appeal can be distributed.

• Participants must make a substantial time commitment.

• A particular experience is required.

• The target population is geographically dispersed.

• Willing participation is especially important.

Methodological judgement

Its main weaknesses are:

• Self-selection.

• Topic interest.

• Strong opinions.

• Advertisement placement.

• Unequal access to the appeal.

• Limited representativeness.

• Weak generalisation.

Balanced evaluation

Volunteer sampling may allow researchers to recruit many willing participants through one advertisement. This is useful when the study requires considerable time or repeated attendance. However, people who respond may be more interested, confident or motivated than those who do not. Their responses may therefore differ from the wider population, producing volunteer bias and limiting generalisation.

Improving opportunity sampling

Researchers may reduce bias by:

• Recruiting at several locations.

• Recruiting at different times and on different days.

• Approaching every eligible person rather than selecting preferred individuals.

• Using a fixed selection rule.

• Recording how many people refuse.

• Comparing the sample’s characteristics with the population.

• Repeating the study with another sample.

These changes can reduce bias but do not transform the procedure into random sampling.

Improving volunteer sampling

Researchers may:

• Advertise through several channels.

• Use neutral advertisement wording.

• Provide accessible response methods.

• Avoid excessive incentives.

• State eligibility criteria clearly.

• Record the number and characteristics of respondents.

• Randomly select from eligible volunteers when too many respond.

• Replicate the study with another sample.

Random selection within a volunteer pool can reduce researcher choice among volunteers, but it does not remove volunteer bias.

Worked scenario 1: opportunity sampling

A psychologist wants to investigate concentration among all students at a college.

She recruits 25 students who are available in the Psychology department during a free period.

Method

Opportunity sampling.

Possible sources of bias

The sample may overrepresent students who:

• Study Psychology.

• Spend free periods in that department.

• Have no scheduled lesson.

• Are known to the researcher.

• Are comfortable participating in psychological research.

Effect on generalisation

The findings may not represent:

• Students studying other subjects.

• Students who spend free periods elsewhere.

• Students unavailable at that time.

• The whole college population.

Possible improvement

Recruit students from:

• Several departments.

• Different times of day.

• Both year groups.

The researcher could also use a predetermined rule for approaching eligible students.

Worked scenario 2: volunteer sampling

A researcher posts an advertisement on a sleep-support website asking adults to complete a sleep questionnaire.

Method

Volunteer sampling.

Possible sources of bias

Respondents may:

• Have sleep difficulties.

• Be especially interested in sleep.

• Use online support resources.

• Have internet access.

• Feel motivated to discuss the topic.

Effect on findings

The researcher may overestimate sleep problems if the findings are generalised to all adults.

Possible improvement

Advertise through:

• General community channels.

• Workplaces.

• Educational settings.

• Offline and online locations.

The researcher should still acknowledge that the sample remains self-selected.

Worked scenario 3: distinguishing the methods

Scenario A

A researcher stands outside a college café and asks the first 20 eligible students who pass to participate.

This is opportunity sampling because the researcher approaches available people.

Scenario B

A researcher places a poster outside the college café and recruits the first 20 eligible students who email.

This is volunteer sampling because students respond to an appeal.

Scenario C

A researcher emails 100 randomly selected students and 20 agree to participate.

The original selection is random, although non-response may affect the final sample.

It is not automatically volunteer sampling merely because selected participants can refuse.

Opportunity and volunteer sampling compared with other methods

Compared with random sampling

Random sampling gives every member of an appropriate sampling frame an equal chance of selection.

Opportunity and volunteer sampling do not.

They are generally:

• Faster.

• Easier.

• Less dependent on population lists.

They are also generally more vulnerable to sampling bias.

Compared with systematic sampling

Systematic sampling selects every nth person from an ordered list.

Opportunity sampling selects whoever is available.

Volunteer sampling selects those who respond.

Systematic sampling may reduce researcher choice, but it requires an organised sampling frame.

Compared with stratified sampling

Stratified sampling represents relevant population groups proportionately.

Opportunity and volunteer samples may accidentally:

• Overrepresent some groups.

• Underrepresent others.

• Exclude groups completely.

Stratification is more complex but may support stronger generalisation.

A method for identifying the sampling method

Question 1: Did the researcher approach available people?

If yes, this indicates opportunity sampling.

Look for phrases such as:

• Available at the time.

• Present in the location.

• First people encountered.

• Members of an accessible class.

• People nearby.

Question 2: Did people respond to a general appeal?

If yes, this indicates volunteer sampling.

Look for phrases such as:

• Responded to an advertisement.

• Replied to an email.

• Signed up through a website.

• Answered a poster.

• Put themselves forward.

Question 3: Was everyone selected by chance?

If yes, this may be random sampling rather than either convenient method.

Question 4: Was every nth person selected?

If yes, this indicates systematic sampling.

A method for evaluating a sampling scenario

Step 1: identify the target population

State who the researcher wants to understand.

Step 2: identify the sample

State who actually participated.

Step 3: name the sampling method

Use the selection procedure described.

Step 4: identify the source of bias

For opportunity sampling, consider:

• Location.

• Time.

• Availability.

• Researcher choice.

For volunteer sampling, consider:

• Interest.

• Self-selection.

• Advertisement access.

• Strong opinions.

Step 5: link the bias to the measured behaviour

Explain why the overrepresented characteristic could affect the result.

Step 6: judge generalisation

State which wider groups are not represented adequately.

Step 7: suggest a realistic improvement

Change the recruitment procedure rather than simply saying “use more people”.

Writing an effective definition

Opportunity sampling

Opportunity sampling is a method in which researchers recruit people who are available and willing to participate at a convenient place and time.

Volunteer sampling

Volunteer sampling is a method in which people select themselves for research by responding to an advertisement or invitation.

Writing an effective comparison

In opportunity sampling, the researcher selects from people who are currently available. In volunteer sampling, potential participants decide to put themselves forward after seeing an appeal. Opportunity samples are therefore particularly affected by location and researcher choice, whereas volunteer samples are particularly affected by self-selection.

Writing an effective opportunity-sampling evaluation

One strength of opportunity sampling is that it is quick because participants are already present and accessible. The researcher does not need a complete sampling frame or lengthy recruitment process. However, people available in one location may differ from the target population. For example, students recruited from a library may revise more frequently than other students, limiting generalisation of findings about revision behaviour.

Writing an effective volunteer-sampling evaluation

One strength of volunteer sampling is that respondents have actively expressed interest in participation, which may make recruitment for a demanding study easier. However, volunteers may be more interested in the topic or have stronger opinions than non-volunteers. Their responses may therefore be unrepresentative of the wider population, producing volunteer bias.

Overall summary

Opportunity sampling

Opportunity sampling:

• Uses people who are available.

• Is initiated by the researcher.

• Usually occurs at a convenient time and location.

• Is quick, inexpensive and practical.

• Does not require a complete sampling frame.

Its main limitations are:

• Availability bias.

• Location and time bias.

• Researcher selection.

• Poor representativeness.

• Limited generalisation.

Volunteer sampling

Volunteer sampling:

• Uses people who respond to an advertisement or appeal.

• Is initiated by potential participants.

• Can reach many willing people.

• May be useful for demanding or specialised research.

• Does not require direct recruitment of every participant.

Its main limitations are:

• Self-selection.

• Topic-interest bias.

• Strong-opinion bias.

• Advertisement-channel bias.

• Poor representativeness.

• Limited generalisation.

The key distinction is:

Opportunity participants are selected because they are available, whereas volunteers select themselves by responding to an invitation.

Key Words 🔑

Key word	Student-friendly definition	How it may be used in an exam
Opportunity sampling	Selecting people who are available and willing at a convenient place and time.	Identify or describe direct recruitment of accessible participants.
Volunteer sampling	Recruiting people who put themselves forward after an invitation or advertisement.	Identify or describe self-selected participation.
Self-selection	The process through which people choose to enter a study themselves.	Explain the defining feature of volunteer sampling.
Target population	The complete group about which the researcher wants to draw conclusions.	Compare the wider group with the opportunity or volunteer sample.
Sample	The smaller group that participates and provides the research data.	Identify who actually took part.
Sampling bias	A systematic difference between the sample and the target population.	Explain why findings may not generalise.
Opportunity bias	Bias caused by recruiting only people available in a particular setting or period.	Evaluate an opportunity-sampling procedure.
Volunteer bias	Bias caused when volunteers differ systematically from non-volunteers.	Evaluate a self-selected sample.
Researcher selection bias	Bias resulting from the researcher choosing particular available people.	Explain why opportunity samples may be unrepresentative.
Location bias	Bias caused by recruiting participants from a particular place.	Apply evaluation to a school, shop, library or workplace.
Time bias	Bias caused by recruiting participants only during a particular period.	Explain why some population members were unavailable.
Advertisement bias	Bias caused by the content or location of a recruitment appeal.	Evaluate who was likely to see and answer an advertisement.
Representative sample	A sample reflecting relevant characteristics of its target population.	Judge whether generalisation is justified.
Generalisability	The extent to which findings can be applied beyond the sample.	Explain the consequence of sampling bias.
Sampling frame	An organised list of population members from which participants may be selected.	Explain why opportunity and volunteer sampling may be practical without one.
Random sampling	Selecting participants through chance so every listed population member has an equal chance.	Compare objective selection with convenience or self-selection.
Systematic sampling	Selecting every nth person from an ordered population.	Compare a fixed procedure with opportunity recruitment.
Stratified sampling	Selecting population groups in proportion to their size.	Compare representativeness with convenient sampling.
Non-response bias	Bias occurring when participants differ from selected people who refuse.	Evaluate the final sample after invitations.
Informed consent	Agreement to participate after receiving sufficient information.	Explain that convenient recruitment does not remove ethical duties.

Common Mistakes ⚠️

Mistake: Saying opportunity sampling means asking for volunteers.

Why this is incorrect:In opportunity sampling, the researcher approaches people who are available. Volunteers respond to a general appeal.

How to improve:Identify who initiates the recruitment.

Mistake: Saying volunteer sampling means selecting people who are willing after being randomly chosen.

Why this is incorrect:All participants can refuse, but volunteer sampling specifically begins when people respond to an advertisement or request.

How to improve:Look for self-selection from a general invitation.

Mistake: Describing opportunity sampling only as “easy”.

Why this is incomplete:A developed explanation should state that accessible participants can be recruited without a complete sampling frame.

How to improve:Link convenience to reduced time, cost or administration.

Mistake: Saying opportunity sampling has no procedure.

Why this is incorrect:Researchers should define the location, time, eligibility rules and way available people are approached.

How to improve:Describe a clear and replicable recruitment procedure.

Mistake: Saying opportunity participants have no choice about participation.

Why this is incorrect:They must still provide consent and may refuse.

How to improve:Distinguish being approached from being forced to take part.

Mistake: Saying volunteers are representative because everyone could respond.

Why this is incorrect:Not everyone will see the appeal, have access to it or feel motivated to answer.

How to improve:Consider self-selection and the advertisement channel.

Mistake: Assuming a large volunteer sample has no bias.

Why this is incorrect:Thousands of respondents can still differ systematically from non-volunteers.

How to improve:Evaluate both sample size and selection method.

Mistake: Naming volunteer bias without explaining it.

Why this is incomplete:The examiner needs to know how volunteers may differ from non-volunteers.

How to improve:Refer to interest, confidence, availability, strong opinions or motivation.

Mistake: Naming location bias without linking it to the research topic.

Why this is incomplete:A location matters when its users differ in a way that may affect the findings.

How to improve:Explain, for example, why students in a library may have different revision habits.

Mistake: Claiming that opportunity or volunteer sampling always produces invalid findings.

Why this is incorrect:The data may accurately describe the participants tested.

How to improve:Explain that the main problem concerns representativeness and generalisation.

Mistake: Saying random selection from volunteers creates a random sample of the population.

Why this is incorrect:Only people who volunteered could be selected.

How to improve:Describe it as random selection from a volunteer pool and acknowledge remaining volunteer bias.

Mistake: Ignoring the wording of a volunteer advertisement.

Why this is incorrect:Leading or emotive wording may attract people with particular experiences or opinions.

How to improve:Use neutral wording and distribute the appeal through several channels.

Mistake: Generalising from one convenient setting to the whole population.

Why this is incorrect:People at one location may share characteristics or experiences not found elsewhere.

How to improve:Restrict conclusions or recruit across several settings.

Mistake: Suggesting a larger sample as the only improvement.

Why this is incorrect:A larger biased sample remains biased.

How to improve:Change how participants are recruited and improve population coverage.

Mistake: Confusing sampling with allocation to experimental conditions.

Why this is incorrect:Sampling determines who enters the study. Allocation determines which condition participants enter.

How to improve:Evaluate recruitment separately from random allocation or the experimental design.

Exam-Style Questions ✍️

Question 1

Which one of the following best describes opportunity sampling?

A. Selecting every tenth person from a list

B. Recruiting people who are available at the time

C. Selecting population groups proportionately

D. Recruiting only those who answer an advertisement

[1 mark]

Question 2

Define volunteer sampling.

[2 marks]

Question 3

Explain one difference between opportunity sampling and volunteer sampling.

[3 marks]

Question 4

A psychologist recruits 30 students who are sitting in the college library during lunchtime.

Identify the sampling method and explain your answer.

[3 marks]

Question 5

A researcher places an online advertisement asking people to complete a questionnaire about anxiety.

Explain one way in which the resulting sample may be biased.

[4 marks]

Question 6

Explain one strength and one limitation of opportunity sampling.

[6 marks]

Question 7

Explain one strength and one limitation of volunteer sampling.

[6 marks]

Question 8

A researcher recruits 5,000 volunteers through a social-media page about sleep difficulties. She concludes that the results represent all adults.

Explain why this conclusion may not be justified.

[4 marks]

Question 9

A psychologist wants to investigate revision behaviour across a sixth-form college. He recruits students from an optional after-school revision workshop.

Explain how the recruitment procedure could be improved to reduce sampling bias.

[4 marks]

Question 10

Evaluate opportunity and volunteer sampling as methods of selecting participants for psychological research.

Refer to sampling bias, representativeness and generalisation in your answer.

[8 marks]