What is Cronbach's alpha?

Also called Coefficient alpha, Internal consistency

Cronbach's alpha is an index of internal consistency: how far the items in a scale behave as though they measure one thing. It runs from 0 to about 1 and rises with both the average correlation between items and the sheer number of items, which is why a high alpha on its own proves very little.

How to read a reported alpha

The conventional rules of thumb — 0.70 acceptable, 0.80 good, 0.90 excellent — are widely quoted and were never intended as thresholds. What matters is the decision the score supports: an alpha that is comfortable for a research scale is not comfortable for a scale used to reject an individual candidate.

Alpha above roughly 0.95 is usually a warning rather than a triumph. It generally means items are near-duplicates of each other, so the scale is long, narrow and redundant, and it measures a smaller slice of the construct than its name suggests.

Why the length effect matters commercially

Because alpha rises with item count, a vendor can raise it simply by adding paraphrases of existing questions. This makes alpha the easiest reliability figure to inflate and the least informative to compare across instruments of different lengths.

The more defensible modern practice is to report McDonald's omega alongside or instead of alpha, because omega does not assume every item contributes equally to the underlying factor — an assumption that is almost never true of real questionnaires.

What the item count does on its own

Average inter-item correlation 0.30, 5 items → alpha ≈ (5 × 0.30) ÷ (1 + 4 × 0.30) = 0.68
Same 0.30 correlation, 10 items → alpha ≈ (10 × 0.30) ÷ (1 + 9 × 0.30) = 0.81
Same 0.30 correlation, 20 items → alpha ≈ 0.90

Nothing about the quality of the items changed between those three lines. Ask for the item count whenever an alpha is quoted, and be sceptical of a high figure on a long scale.

Not the same as test–retest reliability

Alpha describes agreement between items within a single sitting. Test–retest describes agreement between two sittings by the same people. A scale can be internally consistent and still give a different answer next month.

Test–retest reliability