Sampling, validity and reliability
Define population and sample, compare opportunity and volunteer sampling, and evaluate validity and reliability within syllabus limits.
Section Paper 2 · Paper 2 · Topics 4–6 and non-experimental methods
Original teaching mapped to Cambridge IGCSE Psychology 0266 Version 3 for 2027–2029. The course is independent study support, not Cambridge material, an official mark scheme or an awarded grade. Named-study notes summarise the required findings; candidates are not expected to evaluate named studies. Self-check practice is not official grading, a timed Paper 1/2 simulator or teacher review.
Before you begin
- Use the Version 3 syllabus and confirm availability in your administrative zone with Cambridge and your school. Both papers are compulsory. The updated named-study summaries are required context; full original research articles are not required.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before sampling, validity and reliability, which approach is most useful?
What you will learn
- Distinguish population and sample
- Compare opportunity and volunteer sampling
- Explain validity, subjectivity and objectivity
- Explain reliability and inter-rater/inter-observer agreement
A sample represents a target population
A population is the group sharing characteristics relevant to a study; a sample is the people selected to represent it. Opportunity sampling uses people available at the time. Volunteer sampling uses people who respond to an invitation. The syllabus limits named techniques to these two.
Validity asks whether the aim is being measured
Validity is the extent to which the research investigates what it set out to investigate. Subjective judgements are affected by personal opinions; objective judgements are less affected. Improve validity by checking that the operational definition and materials genuinely represent the aim. Named validity subtypes are outside the syllabus.
Reliability asks whether results are consistent
Reliability is consistency of procedure or measures. Inter-rater reliability is agreement between people judging the same responses; inter-observer reliability is agreement between observers recording the same behaviour. Clear instructions and operational definitions can improve consistency. Calculating reliability coefficients is not required.
Pause and explain
Which response best shows understanding of sampling, validity and reliability?
Worked example
A researcher invites volunteers to study and then recruits whoever replies. Identify the technique and one likely concern.
Show the worked solution
- Participants respond to an invitation.
- This is volunteer sampling.
- Those who choose to reply may differ from people who do not.
Answer Volunteer sampling; the sample may not represent people who do not volunteer.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A researcher asks students in the next classroom because they are easiest to reach. Identify the sample technique.
Give me a hint
Availability drives selection.
Compare my reasoning
- Students are selected because they are available.
- The researcher did not invite a wider group to volunteer.
- This is opportunity sampling.
Opportunity sampling.
Look for these in your work
- Identifies availability
- Distinguishes invitation
- Names the method
A coding scheme produces different behaviour counts when used by two observers. Which quality is weak, and how could it improve?
Give me a hint
Agreement across observers is the key.
Compare my reasoning
- The observers record the same behaviours.
- Different counts indicate weak inter-observer reliability.
- Clarify categories and practise with common examples before the study.
Inter-observer reliability is weak; improve it with clear operational categories and observer practice.
Look for these in your work
- Identifies reliability type
- Explains disagreement
- Proposes a relevant improvement
A questionnaire claims to measure exam anxiety but asks only how many hours students study. Explain the validity issue.
Give me a hint
Does the measure match the intended construct?
Compare my reasoning
- Study time is measured directly.
- Exam anxiety is the stated aim.
- The measure may not represent anxiety, weakening validity.
The questionnaire may lack validity because study hours do not directly measure exam anxiety.
Look for these in your work
- Identifies measure
- Identifies aim
- Explains mismatch
Common mistakes
- Confusing random allocation with random sampling.
- Using ‘valid’ to mean merely repeatable.
- Trying to calculate inter-rater coefficients not required by the syllabus.
What can you explain now?
Which type of reliability concerns two observers recording the same behaviour?
Compare with the explanation
Inter-observer reliability
Inter-rater reliability concerns judgments of the same responses.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do population, sample, validity and reliability answer different questions about research quality?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.