JAMAevidence Glossary

Terms are derived from Users' Guides to the Medical Literature: A Manual for Evidence-Based Practice, 2nd Edition and The Rational Clinical Examination: Evidence-Based Clinical Diagnosis. Updated December 2009.
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Validity
In health status measurement terms, validity is the extent to which an instrument measures what it is intended to measure. In critical appraisal terms, validity reflects the extent to which the study results are likely to be subject to systematic error and thus be more or less likely to reflect the truth. See also Credibility.

Values
When used generically, as in “values and preferences,” we refer to the collection of goals, expectations, predispositions, and beliefs that individuals have for certain decisions and their potential outcomes. The incorporation of patient values and preferences in decision making is central to evidence-based medicine. These terms also carry specific meaning in other settings. Measurement tools that require a choice under conditions of uncertainty to indirectly measure preference for an outcome in health economics (eg, the standard gamble) quantify preferences. Measurement tools that evaluate the outcome on a scale with defined favorable and unfavorable ends (eg, visual analog scales, feeling thermometers) quantify values.

Variance
The technical term for the statistical estimate of the variability in results.

Variant allele
The allele at a particular SNP that is the least frequent in a population.

Verification bias
Results of a diagnostic test influence whether patients are assigned to an intervention group. See also Differential verification bias.

Visual analogue scale
A scaling procedure consisting of a straight line anchored on each end with words or phrases that represent the extremes of some phenomenon (eg, “worst pain I have ever had” to “absolutely no pain”). Respondents are asked to make a mark on the line at the point that corresponds to their experience of the phenomenon.