Psychometric tests and criteria (adapted from Lamping et al8)
| Psychometric property | Definition | Criteria |
|---|---|---|
| 1 Item reduction | Identify items for possible elimination | Applied to each of the 45 items |
| Missing data <5% | ||
| No item redundancy (inter-item correlations <0.80) | ||
| Item-total correlations ⩾0.20 | ||
| Maximum endorsement frequencies <80% including floor/ceiling effects (that is, response categories with high endorsement rates at the bottom/top end of the scale respectively) | ||
| Aggregate adjacent endorsement frequencies ⩾10% | ||
| Item convergent/discriminant analyses (items must be scaling successes or probable scaling successes) | ||
| 2 Acceptability | Completeness of data and score distributions | Applied to each of the 33 items |
| Missing data <5% | ||
| Maximum endorsement frequencies <80% including floor/ceiling effects | ||
| Applied to summary scores | ||
| Missing data <5% | ||
| Floor/ceiling effects <80% | ||
| Skewness values between +1 to −1 | ||
| 3 Reliability | ||
| 3.1 Internal consistency | The extent to which items comprising a scale measure the same construct | Cronbach α coefficients for summary scores >0.70 |
| Item-total correlations ⩾0.40 | ||
| 3.2 Test-retest reliability | The stability of an instrument assessed by administering the instrument to respondents on two separate occasions | Pearson/Spearman correlations >0.80 |
| 4 Validity | ||
| 4.1 Content validity | Extent to which content of instrument or scale is representative of intended conceptual domain | Content derived from focus groups and field testing |
| 4.2 Construct validity | ||
| 4.2.1 Within scale analyses | Evidence that a single construct is being measured | Internal consistency Cronbach α coefficient >0.70 |
| Item-total correlations ⩾0.25 | ||
| 4.2.2 Analyses against external criteria | ||
| 4.2.2.1. Known group differences | Evidence that the instrument differentiates between groups who are known to differ—eg, by presence or severity of disease | Expected higher IND-VFQ scores in patients with eye disease compared to normals |
| 4.2.2.2 Convergent validity | Evidence that the instrument correlates with measures of the same of a similar construct | Expected correlation with visual acuity |
| 4.2.2.3 Discriminant validity | Evidence that the instrument is not correlated with measures of different constructs | Expected lack of association with age and sex |
| 5 Responsiveness | Ability of a scale to detect clinically significant change following a treatment of known efficacy | Expected improved scores after cataract surgery |
| Effect sizes (calculated as mean difference in scores divided by pooled standard deviation at baseline) compatible with others published for similar treatment |