IB Psychology HLTopic 5 — Research DesignPaper 3 & IACore idea~11 min read
Validity: Measuring What You Meant To
Reliability asks whether you would get the same answer again. Validity asks the harder question: was it the right answer to the question you were actually asking? A study can be beautifully controlled, perfectly repeatable, and still be measuring something nobody cares about.
📚 What you need to know
Validity is the extent to which a study measures what it claims to measure and reflects real behaviour.
Internal validity: was the result really caused by the IV, and not by extraneous or confounding variables?
External validity: do the findings apply beyond this particular study?
Ecological validity is a type of external validity about how realistic the task and setting are.
Temporal validity asks whether the findings still hold in a different era.
Construct validity asks whether the measure really captures the psychological idea it claims to.
Predictive validity asks whether the findings successfully forecast future behaviour.
The dartboard picture
The quickest way to hold reliability and validity apart is to picture throwing darts. Reliability is how tightly your darts cluster. Validity is whether that cluster is on the bullseye. Those are two separate questions, and you can have one without the other.
The middle board is exactly the broken thermometer: consistent to the millimetre, and pointing at the wrong place every time.
Internal and external: inside the study, outside the study
These two are the backbone of every evaluation you will write. Internal validity lives inside the study: it asks whether the result really came from the IV. Confounding variables, demand characteristics and investigator effects all attack it. External validity lives outside: it asks whether the finding travels to other people, other places and other times.
Almost every method evaluation in IB Psychology is really an argument about where a study sits on this trade-off.
The named types you have to know
Type
The question it asks
A study that shows it
Internal
Was the change in the DV caused by the IV and nothing else?
Asch used identical line stimuli and the same seating position on every trial
External
Do the findings apply beyond this sample and setting?
Dickerson studied prosocial behaviour in a genuine real-world setting
Ecological
Is the task and setting realistic enough to reflect everyday behaviour?
Boyden documented the lived experience of children in war zones
Temporal
Do the findings still hold today?
Asch’s conformity results reflect a post-war social climate that has since changed
Construct
Does the measure capture the psychological idea it claims to?
Asch’s task was unambiguous, so conforming could only mean conformity
Predictive
Do the findings forecast later behaviour?
Ainsworth’s attachment types were used to predict later relationship patterns
Construct validity is the one students skip, and it is the most useful in the IA. If you claim to measure “stress” with a three-question survey you invented, the marker’s first thought is: does that actually measure stress? Answer it before they ask.
Ecological validity is not just about location. A study can happen in a real cafe and still be ecologically weak if the task is artificial. It can happen in a lab and be ecologically strong if participants are genuinely engaged in something that matters to them.
🧩 Attacking validity in an evaluation
Start internal. Name a specific confounding variable that could explain the result instead of the IV.
Then construct. Ask whether the DV really measured the concept in the hypothesis.
Then ecological. Ask whether anybody does this task in real life.
Then sample. Who was studied, and who is being left out of the conclusion?
Then time. Would the result look the same if the study were run today?
Finish with a fix that would raise one of them, and say which one.
Worked examples
WORKED EXAMPLE
Identify the threat to validity
Students memorise a list of 20 unrelated words in a silent lab, then recall as many as they can. The researcher concludes that people can hold about seven items in short-term memory in everyday life. Identify the main threat to validity and explain it.
Step 1: Check internal validity first
The setting is well controlled and the IV is manipulated cleanly, so internal validity is high.
Step 2: Find where the conclusion overreaches
The claim is about everyday life, but nobody memorises unrelated word lists in everyday life.
Step 3: Name the threat
Low ecological validity, and therefore weak external validity for the conclusion drawn.
Step 4: Suggest an improvement
Use meaningful material such as a shopping list or a set of directions, so the task resembles real remembering.
Low ecological validity limits the everyday claimthe flaw is in the conclusion, not the procedure — say so
WORKED EXAMPLE
Explain a confounding variable
An experiment tests whether caffeine improves reaction time. The caffeine group is tested at 9am and the water group at 4pm. The caffeine group is faster. Explain why the conclusion cannot be trusted.
Step 1: Identify the extra variableTime of day differs between the two conditions.
Step 2: Show that it varies with the IV
Because it changes systematically alongside the IV, it is not merely extraneous but confounding.
Step 3: Give the rival explanation
Participants may simply be more alert in the morning, so the faster times could have nothing to do with caffeine.
Step 4: Name the damage and the fixInternal validity is low. Test both conditions at the same time of day, or counterbalance the testing times across groups.
Time of day is confounded with the IV; internal validity is destroyedto be confounding, the variable must line up with the conditions
💡 Exam tip
Never write “the study lacks validity”. Say which validity, and why.
A confounding variable point earns most credit when you name the specific variable and give the rival explanation.
Use the control-versus-realism trade-off. It is a ready-made evaluation paragraph for almost any experiment.
Temporal validity is easy marks on any older classic study. Say what has changed socially since.
For the IA, one sentence on construct validity lifts the evaluation section noticeably.
Keep the direction right: reliability is needed for validity, but validity is the bigger claim.
⚠ Common mix-up
Using validity and reliability interchangeably. Consistent is not the same as correct.
Equating ecological validity with an outdoor setting. It is about how realistic the task is, not the postcode.
Treating high internal validity as an all-round strength. It usually comes at a cost to external validity.
Calling every nuisance variable confounding. It only counts once it varies systematically with the IV.
Forgetting construct validity exists. It is the one that questions whether your measure means anything.
Saying a large sample raises validity. It mainly helps generalisability, which is only one part of external validity.
Up next: Generalisability: Who Else Do the Results Apply To? — taking external validity apart and asking exactly how far a finding is allowed to travel.
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