What a self-report test can say about you
A questionnaire observes nothing: it records a self-description. That bounds what can be concluded from it, and it does not make it useless.
Two regimes of measurement
Differential psychology distinguishes maximal performance, measured by tasks with correct answers, from typical performance, measured by what the person reports of their usual conduct. An aptitude test belongs to the first regime, a questionnaire to the second.
Confusing the two is the source of just about every misunderstanding about tests. A questionnaire does not measure a capacity, however carefully it is built, and no statistical treatment turns a declaration into an observed performance.
What distorts a declaration
Three biases are well documented. Acquiescence is the tendency to agree whatever the question, corrected by phrasing some items in the opposite direction. Extreme response style is the preference for the ends of the scale rather than the middle, and it varies strongly from one population to another.
The third is the most insidious: the reference group effect. Declaring oneself rather calm presupposes a comparison with those around one, who are not the same for everyone. Two equally calm people, one surrounded by reserved people, the other by demonstrative ones, will not answer the same thing.
Memory is not an average
Questions bearing on a past period assume the memory is faithful. The work of Daniel Kahneman and colleagues shows that it is not: the retrospective judgement of an episode depends mainly on its most intense moment and on its ending, and very little on its duration.
A questionnaire therefore inherits this distortion. It is one of the reasons why the best-built items bear on typical situations rather than on periods to be evaluated globally.
What the declaration catches all the same
Despite these limits, self-report measures have two useful properties. They are stable: the same person answers similarly several months apart. And they are organised: the answers are not distributed at random but cluster into dimensions recovered from one sample to another.
They also show moderate agreement with descriptions made by close acquaintances, weaker than one would wish, stronger than suspicion would have it. A self-description is neither the truth nor a fiction: it is one source among others, with its own errors.
Two reading rules
First rule: a gap of a few points between two dimensions is almost always below the instrument's margin of error, and therefore uninterpretable. A serious report comments on clear positions and ignores minimal gaps.
Second rule: a self-report test suits a self-knowledge exercise, where the person has no interest in flattering themselves, and suits a recruitment decision badly, where they do. The studies show it unambiguously: ask someone to present themselves in their best light, and their score rises.
Key points
A questionnaire measures a self-description, never an observed aptitude.
Acquiescence, response style and the reference group effect weigh on every declaration.
These measures are stable and structured, which makes them useful for self-knowledge and unsuited to a hiring decision.
This article describes documented mechanisms and disagreements. It recommends no position, no party and no vote, and commits to no reading other than that of the work it cites.
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