Measurement Reliability and Validity

“A scale can repeat the same error every time: consistency is not correctness.”

01 / PLAINLY

What it means, plainly

Reliability asks whether a measure is consistent. Validity asks whether it is suitable for representing what it claims to measure.

02 / CONTEXT

A little more

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 / WHY IT MATTERS

Why it matters

It keeps a repeated number from earning trust by repetition alone and asks whether the measure fits the decision at hand.

04 / EXAMPLE

A familiar example

A scale that adds two kilograms every time is repeatable, but not valid for estimating weight unless its bias is corrected.

05 / LIMIT

What it does not mean

High agreement across repetitions does not prove that the right construct is being measured.

06 / NOTICE

Notice it in your day

Imagine a ruler whose zero mark is shifted. Decide which consistency test it might pass and which measurement-purpose test it would fail.

FINAL NOTE

The idea worth keeping

Check stability, systematic error, and fitness for purpose separately.

QUESTIONS / 02

Questions people still have

Is a reliable measure automatically valid?

No. It may repeat the same error consistently.

What question does validity answer?

Whether the measure is suitable for what it claims to measure.