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# Questionnaire validation

- Concept

Checking that a questionnaire measures consistently (reliability) what it claims to measure (validity) and picks up change.

Updated Nov 13, 2026

## Key points

- Feasibility: completion time, understanding and the percentage of items left unanswered.
- Internal consistency: Cronbach's alpha ≥ 0.70 for group comparisons; > 0.95 suggests redundant items.
- Test-retest: intraclass correlation coefficient (ICC) ≥ 0.70 over an interval in which the condition does not change.
- Content validity (experts and patients), construct validity (expected links with other measures) and criterion validity (against a reference standard).
- Responsiveness: the score moves when the condition improves or worsens.
- Cross-cultural adaptation: translation by two people, synthesis, back-translation, expert committee and cognitive testing.

On this page

- The basics Key points
- What it is
- Key facts
- Worked example
- Measurement properties of a questionnaire
- Markers
- In practice
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

Validating a questionnaire means gathering evidence that its scores are reliable, valid and responsive in a given population. Reliability covers internal consistency (Cronbach's alpha), stability over time (test-retest) and agreement between observers. Validity covers content, construct and criterion validity. An instrument validated in another language or country is not simply translated: it needs cross-cultural adaptation and a new check.

## Key facts

- Cronbach's alpha = [k / (k − 1)] × (1 − sum of item variances / variance of the total score), with k items.
- Reliable is not the same as valid: a questionnaire can always give the same result and still measure something else.

## Worked example

- Data: a 4-item scale given to 120 people. Item variances: 1.0, 1.2, 0.8 and 1.0. Variance of the total score: 10.0.
- Sum of the item variances: 1.0 + 1.2 + 0.8 + 1.0 = 4.0.
- Alpha = [4 / (4 − 1)] × (1 − 4.0 / 10.0) = 1.333 × 0.60 = 0.80.
- Reading the alpha: 0.80 is acceptable internal consistency (≥ 0.70) with no sign of redundancy (> 0.95).
- Test-retest: 40 people answer again after 2 weeks; ICC = 0.85, good stability.
- Criterion: against pill counts, the score correlates at 0.62, a moderate correlation.
- Reading: the scale is reliable and shows criterion validity in this sample; to use it in another country it would need cross-cultural adaptation.

## Measurement properties of a questionnaire

| Property | What it shows | How it is assessed |
| --- | --- | --- |
| Internal consistency | The items measure the same thing | Cronbach's alpha |
| Test-retest | Stability over time | ICC; kappa for categorical answers |
| Inter-rater | Agreement between interviewers | ICC or kappa |
| Content validity | Covers every aspect of the concept | Judgement of experts and patients |
| Construct validity | Behaves as theory predicts | Expected correlations, factor analysis |
| Criterion validity | Agrees with a reference standard | Correlation, sensitivity and specificity |
| Responsiveness | Detects improvement or worsening | Change in score before and after |

## Markers

- Cronbach's α
- ICC
- Kappa

## In practice

- Many quality-of-life, depression and adherence scales have versions validated in Colombia: use the validated version, not a home-made translation.
- A score with no proven validity in the study population weakens the whole result.

## Common pitfalls

- Translating a questionnaire and using it without adaptation or a new validation.
- Presenting a high alpha as proof of validity.
- Running test-retest over an interval so long that the condition has already changed.

## Memory aid

‘Steady is not accurate’: reliability says the shots land in the same place; validity says that place is the bull's-eye.

## Test yourself

Think of the answer, then tap the question to see it.

## A 5-item scale has a sum of item variances of 5 and a total variance of 15. What is alpha?

(5/4) × (1 − 5/15) = 1.25 × 0.667 ≈ 0.83.

## What does a Cronbach's alpha of 0.97 suggest?

That some items are redundant: they ask almost the same thing.

## What does test-retest assess?

Stability: whether the questionnaire gives the same result when the condition has not changed.

## What are the steps of a cross-cultural adaptation?

Independent translation, synthesis, back-translation, expert committee review and cognitive testing with people from the population.

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## Connections

## Related 5

- [Validity and reliability](https://durgical.com/en/epidemiology/validity-and-reliability/)
- [Agreement and the kappa statistic](https://durgical.com/en/epidemiology/agreement-and-the-kappa-statistic/)
- [Questionnaire design](https://durgical.com/en/epidemiology/questionnaire-design/)
- [Reference standard (gold standard)](https://durgical.com/en/epidemiology/reference-standard/)
- [Correlation](https://durgical.com/en/epidemiology/correlation/)

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Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [COSMIN Taxonomy of Measurement Properties](https://www.cosmin.nl/tools/cosmin-taxonomy-measurement-properties/) (COSMIN, 2010); [Collecting Data](https://www.cdc.gov/field-epi-manual/php/chapters/collecting-data.html) (Centers for Disease Control and Prevention (The CDC Field Epidemiology Manual), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/questionnaire-validation/