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Measurement Reliability and Validity
“A scale can repeat the same error every time: consistency is not correctness.”
01 / SUMMARY
In short
Reliability concerns consistency across measurements; validity concerns whether a measure provides suitable information about what it claims to measure. They are related but distinct questions.
03 / SCENE
In the wild
A scale that adds two kilograms every time is repeatable, but not valid for estimating weight unless its bias is corrected.
04 / LIMIT
The limit
High agreement across repetitions does not prove that the right construct is being measured.
FINAL NOTE
What remains
Check stability, systematic error, and fitness for purpose separately.