Select samples and types of data
Choose a sampling method and data format that fit the research question, then explain limits on generalisation and interpretation.
Section Paper 2 · Research Methods
Original StudyForA teaching mapped to the Cambridge 2028–2030 syllabus. These lessons are independent study support, not Cambridge material, official mark schemes or awarded grades. No lesson reproduces a research article or Cambridge question.
Before you begin
- This pathway supports the 2028–2030 syllabus only. AS candidates take Papers 1 and 2; A Level candidates also study two of the four specialist options and take Papers 3 and 4. The separate 2027 syllabus is not mapped here.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Participants opt into an online study after seeing an open invitation. Which sample is this?
What you will learn
- Distinguish population and sample
- Compare opportunity, random and volunteer sampling
- Distinguish data type from measurement objectivity
- Apply sampling and data choices to a novel study
A sample is the people studied; a population is the wider group of interest
Opportunity sampling recruits people who are available; it is practical but can over-represent accessible groups. Volunteer (self-selecting) sampling invites people to opt in; motivation may differ from that of non-volunteers. Random sampling selects by chance from a defined sampling frame; it can reduce selection bias if the frame is suitable and selection is carried out correctly, but it may be difficult to organise.
Generalisation asks whether findings can reasonably apply beyond the sample. A large sample does not automatically represent a population. Consider who could be reached, who agreed to participate, and whether relevant groups are missing.
Data type and objectivity answer different questions
Quantitative data are expressed numerically; qualitative data use descriptive, non-numerical material. Objective recording follows a rule intended to limit the observer’s judgment; subjective data depend more on interpretation or personal report. These distinctions are separate: an interview can produce qualitative material, while a rating scale can quantify a subjective judgment.
Choose the data that answer the question. A count can compare frequency, while an open response can reveal how participants explain an experience. A method’s data type does not alone determine validity or quality.
Sources for this lesson
Pause and explain
A participant rates their own worry from 1 to 10. How is the response best classified?
Worked example
A researcher wants to understand how students describe pre-exam worry. Which data type may help, and how might recruitment affect conclusions?
Show the worked solution
- The question asks how students describe an experience, so open responses can provide qualitative data.
- Recruiting only students who volunteer online may attract those with stronger opinions or easier internet access.
- The findings should not automatically be generalised to all students.
Answer Open qualitative responses may fit the question; volunteer recruitment could create self-selection bias and restrict generalisation.
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 recruits the first 20 people leaving a library. Identify the sampling method and one limitation.
Give me a hint
Consider how participants were chosen, not the order in which they left.
Compare my reasoning
- They were selected because they were readily available at one location.
- This is opportunity sampling.
- Library users at that time may differ from the wider population of interest.
Opportunity sampling; the accessible group may not represent all students or all study habits.
Look for these in your work
- Method named
- Recruitment basis explained
- Generalisability limit linked
Classify a spoken interview transcript coded into themes and a 1–7 self-rating of confidence by data format and likely subjectivity.
Give me a hint
Look at whether results are words or numbers, then how judgments are made.
Compare my reasoning
- The transcript is qualitative; theme coding may involve researcher interpretation unless categories are clearly defined.
- The 1–7 scale is quantitative.
- A confidence rating is still the participant’s subjective report.
Transcript: qualitative and potentially interpretive. Rating: quantitative but subjective.
Look for these in your work
- Both formats classified
- Subjectivity considered separately
- Coding limits noted
A town study wants to estimate how often adults use a new bus route. Compare volunteer sampling with random sampling from the town register.
Give me a hint
Compare self-selection and chance selection from a defined frame.
Compare my reasoning
- An open volunteer invitation may over-represent residents who feel strongly about the route.
- A random sample from a complete, current register gives each listed resident a chance of selection.
- A register can still omit residents or contain outdated details, so the sampling frame must be checked.
Random sampling from an adequate register is more likely to avoid volunteer self-selection, but frame coverage and non-response still limit representativeness.
Look for these in your work
- Both recruitment methods compared
- Selection bias distinguished
- Frame and non-response limits stated
Common mistakes
- Calling opportunity sampling random because participants were approached in a random order.
- Assuming random sampling is representative when the sampling frame excludes relevant groups.
- Treating qualitative as synonymous with subjective and quantitative as synonymous with objective.
- Assuming a large sample alone guarantees generalisability.
What can you explain now?
Can quantitative data be subjective?
Compare with the explanation
Yes. A numerical self-rating is quantitative but may reflect a participant’s subjective judgment.
Data format and source of judgment are separate dimensions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do sample selection, data format and subjectivity affect what a psychological study can conclude?
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.