In one line: A reliable measure gives the same answer when nothing has really changed.
Think of a bathroom scale. If it reads 60 kg, then 64 kg, then 61 kg for the same person within a minute, you cannot trust any single reading.
A psychological measure can wobble in exactly the same way. Reliability is the first test of any measure: does it give steady, repeatable results?
Remember: Reliable means repeatable.
Same conditions in, same result out.
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Key idea: Psychologists check whether a measure is steady over time, whether different people using it agree, and whether its parts agree with each other.
Checking reliability
Test-retest
The same people take the same test twice, a while apart.
If nothing has changed and the scores are much the same, the measure is steady over time.
Inter-rater
Two observers rate the same behaviour separately.
If they closely agree, the measure does not depend on who is using it.
Internal consistency
On a questionnaire, questions meant to measure the same thing should get similar answers.
The parts of the measure agree with each other.
Same test, later · Different raters · Items that agree
Take a new checklist for aggression in the playground. The same children are rated two weeks apart and the scores match (test-retest). Two teachers rate the same break time and closely agree (inter-rater). The items on the checklist all point the same way (internal consistency). The measure looks reliable.
Exam tip: Name the kind of reliability, and say how it would be checked.
"Two observers rate the same children and their scores are compared" is a check. "It should be reliable" is not.
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Key idea: A measure wobbles for a reason. Find the reason, and you know how to fix it.
Three causes and their fixes
Vague categories
"Aggressive" means a push to one observer and a loud voice to another.
Fix: write down exactly what counts, and train the observers until they agree.
Changing conditions
One group is tested in a quiet room and another in a noisy hall.
Fix: keep the instructions, the place and the timing the same for everyone.
Too little to go on
One question, or one glance at a child, is easily thrown off by a bad moment.
Fix: use several questions, or watch for longer, and take the average.
Vague · Changing · Thin
On the playground checklist, the first version just said "aggressive behaviour", and the two teachers disagreed. The second version listed hitting, pushing and name-calling. Agreement rose at once.
One condition: A retest only works if the thing being measured has stayed the same.
A child's mood can change in a week, so a different score may be a real change and not a fault in the measure.
Key idea: A measure can give the same answer every time and still be wrong every time.
Go back to the bathroom scale. This one always reads 5 kg too heavy. It is perfectly consistent: the same person gets the same reading every time. It is also wrong every time.
What reliability tells you
- The measure is steady
- Two readings can be compared
- A change in score means something
What it does not tell you
- That the reading is accurate
- That the measure picks up the right thing
- That the conclusions are sound
Remember: An unreliable measure cannot be trusted at all. A reliable one still has to be checked for accuracy.
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How this is asked: Reliability belongs to the concept of measurement, one of the four concepts in the 15-mark question on a study you are given.
For any measure in the study, ask: would it give the same result again?
The pattern
- Say how the behaviour was measured.
- Would two people using the measure agree? Say why or why not.
- Were the conditions the same for everyone?
- Was there enough to go on: more than one question, more than one occasion?
- Bring in a second concept, then conclude.
The trap: "it was not reliable": Say why the measure would wobble.
"The two observers had not agreed what a 4 means" is an explanation. "Not reliable" is a label.
Discuss the following study with reference to two or more of the following concepts: bias, causality, measurement and/or responsibility.
Two researchers wanted to compare how well children play together in two nurseries. Each researcher visited one nursery for a single morning and gave every child a rating from 1 ("plays alone") to 5 ("very cooperative"). They had not agreed beforehand what each number meant. The average rating was 3.9 at the first nursery and 2.6 at the second. They concluded that children at the first nursery are more cooperative.
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