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.

Ventricular tachycardia
A serious rapid heart rate arrhythmia that originates in the ventricles.

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

Vibratory sensation testing
Vibratory sensation is detected with a vibrating tuning fork. Vibration and position sense (proprioception) are associated, as both are mediated through the dorsal columns.

Vicarious posttraumatic growth
The phenomenon of clinician growth that results from witnessing positive sequelae of other people’s experiences of trauma. This may include the clinician’s feelings that his/her own life has been enriched, deepened, or empowered by witnessing the patient’s or family’s posttraumatic growth.

Visual acuity
Sense of visual perception. Normal visual acuity is 20/20, which means that the patient can see at 20 feet distance what a normal person can see at 20 feet. Decreased acuity, such as 20/40 vision, means that the patient can see at 20 feet what the normal person can see at 40 feet.

Visual Analog Scale for the Management of Acute Pain
A measurement instrument for subjective characteristics or attitudes to pain that cannot be directly measured. When responding to an item, respondents specify their level of agreement with a statement by indicating a position along a continuous line between 2 end points.

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.

Visual field
The entire amount of space a patient can see without moving his or her eyes. Also called field of vision.

Vitreous detachment
Posterior vitreous detachment (PVD) involves separation of the posterior vitreous from the retina as a result of vitreous degeneration and shrinkage.

Vitreous hemorrhage
The release of blood from the blood vessels in the vitreous.