Key Idea: Psychology has to turn ideas into things that can be recorded. A good measure is consistent, and it measures what it claims to.
Measuring
- Operationalise: say exactly what is recorded
- Numbers or words; asked or watched
- Direct or indirect; at the time or later
Reliability
- The same result when nothing has changed
- Over time, between raters, among items
- Helped by clear categories and a standard procedure
Validity
- Construct: the right idea
- Internal: a safe conclusion about cause
- External: holds beyond the study
Doubt
- Type I error: a false alarm
- Type II error: a miss
- Reliable is not the same as valid
The words to know
| Word | What it means |
|---|---|
| Construct | An idea, such as stress, that cannot be seen directly |
| Operationalisation | Defining a variable by the exact way it will be measured |
| Self-report | Asking people about themselves |
| Reliability | The consistency of a measure under the same conditions |
| Inter-rater reliability | Different observers rating the same thing closely agree |
| Validity | Whether a measure or study captures what it claims to |
| Construct validity | Whether the measure captures the actual idea |
| Internal validity | Whether an effect is really due to what the researcher changed |
| Ecological validity | Whether findings apply to real-life settings |
| Type I error | Finding an effect that is not real |
What exactly was recorded? Would it come out the same again? Is it the right thing, and how far does it reach?
Exam-style question
Your class practical was an observation. Your class wanted to measure helpfulness. One student dropped a pile of papers in a corridor, and two observers recorded separately whether each of 30 passers-by stopped to help. The two observers agreed on 29 of the 30. Explain the concept of measurement in relation to the observation in your class practical.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Important: Reliable means consistent. Valid means measuring the right thing. A measure can be the first without being the second.
Tap each card to check yourself.
What is operationalisation? Defining a variable by the exact way it will be measured.
Asked or watched: a rating scale filled in by the student? Asked: it is self-report.
Two observers give very different scores for the same child. What is low? Inter-rater reliability.
A happiness survey that asks about money lacks what? Validity: it measures wealth, not happiness.
Would a laboratory finding hold in everyday life? Which validity? Ecological validity.
Type I or type II: a small study misses a real effect? Type II: a miss.
Exam tips
- State the operational definition before judging it.
- For reliability, say who or what would have to agree.
- For validity, name the kind and what threatens it.
- Ask what the measure misses, and what else it could pick up.