Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9990

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.

Cambridge International AS & A Level Psychology 9990 · 2028–2030 · Version 1 · MC-SAMPLING / MC-DATA-TYPES

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?

Put the idea to work

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
  1. The question asks how students describe an experience, so open responses can provide qualitative data.
  2. Recruiting only students who volunteer online may attract those with stronger opinions or easier internet access.
  3. 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.

1 · Guided

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
  1. They were selected because they were readily available at one location.
  2. This is opportunity sampling.
  3. 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
2 · Independent

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
  1. The transcript is qualitative; theme coding may involve researcher interpretation unless categories are clearly defined.
  2. The 1–7 scale is quantitative.
  3. 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
3 · Transfer

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
  1. An open volunteer invitation may over-represent residents who feel strongly about the route.
  2. A random sample from a complete, current register gives each listed resident a chance of selection.
  3. 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.
Recall without your notes

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.

Your review choice appears on Today. Sign in to sync it across devices.

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.

Keep learning Psychology

Course scope and assessment

Seven original lessons map all 15 AS research-method and methodological-concept sections to explained Guided, Independent and Transfer practice. All 12 compulsory AS core studies have original study-specific teaching and staged practice; five additional A Level methodology lessons cover research design and validity. All 15 Clinical, all 15 Consumer (2.1.1–2.5.3) and all 15 Health (3.1.1–3.5.3) Psychology sections link to original teaching and staged practice. All 15 Organisational Psychology sections 4.1–4.5 now have original teaching and explained staged practice; the six new 4.4–4.5 lessons bring the planning map to 92 of 92 indexed sections. Consumer lessons cover physical and psychological environments; decision models, heuristics and mistakes; packaging, selling and buying; advertising, self-monitoring, product placement, brand recognition and slogans. Evaluation boundaries are explicit: studies use particular products, samples, tasks and settings; gaze, recall, self-report, EEG indices, ratings, choices and purchases are different outcomes. Health evidence is bounded: Laba et al. measured hypothetical preferences; Riekert and Drotar raise a non-response concern; Chung and Naya's cap tracked openings, not swallowing; Chaney et al. was a short caregiver-reported pilot; Yokley and Glenwick examined immunisation attendance in one public-health setting. MacLachlan et al. report one patient, not a controlled treatment effect; Brudvik et al. compared separate child, parent and physician ratings in one emergency department. Chandola et al. found a cohort association; Wang et al. measured responses during a monitored arithmetic task; Evans and Wener observed commuters; Bridge et al. found mixed self-rated outcomes in radiotherapy. Janis and Feshbach measured short-term self-reports in one school; Lewin et al. tested a rehabilitation package, not information alone. Tapper et al. and Fox et al. studied bounded school and worksite programmes; incentive effects do not establish transfer or rule out reporting effects. Weinstein measured comparative expectations, while Seligman’s framework is conceptual and Shoshani and Steinmetz used a school-level comparison. These findings do not establish universal health effects or diagnose individuals. Need-theory examples use a child measurement study, not a workplace test; SMART is a practical checklist, not a motivational theory. Landry et al. compared reward framing with student volunteers in a lab; this does not establish universal effects in workplaces. Cuadrado et al. (2008) measured undergraduate evaluations of presented leaders, not workplace performance or hiring; this does not establish a universal gender effect. The historical 1995 Munchausen criteria are not a current diagnostic checklist. Cambridge’s 2028–2030 Reference List treats Oruč et al. (1997) as an example of genetic explanation although its participants had bipolar I; gambling disorder is listed in Cambridge’s Clinical topic but classified under addictive behaviours in ICD-11. Candidates choose two options, but all four remain listed so schools can select a route. The 92-section count is a planning index, not a Cambridge score. Remaining work is deeper coverage review, separate-year syllabus reconciliation, timed-paper practice and human subject-expert review. The separate 2027 and 2024–2026 syllabuses are not reconciled into this route. Practice is not official grading, a timed-paper simulation or human subject-expert review.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.