D

Descriptive study
A  study that describes characteristics of a sample of individuals. Unlike an experimental study, the investigators do not actively intervene to test a hypothesis, but merely describe the health status or characteristics of a sample from a defined population.

Detection bias (synonym: ascertainment bias)
Systematic difference between comparison groups in how outcomes are ascertained, diagnosed or verified.  (Also called ascertainment bias.)

Distribution
The collection of values of a variable in the population or the sample, sometimes called an empirical distribution. See also probability distribution.

Dichotomous data (synonym: binary data)
Data that can take one of two possible values, such as dead/alive, smoker/non-smoker, present/not present. (Also called binary data.) Sometimes continuous data or ordinal data are simplified into dichotomous data (e.g. age in years could become <75 years or ≥75 years).

Dose dependent
A response to a drug which may be related to the amount received (i.e. the dose). Sometimes trials are done to test the effect of different dosages of the same drug. This may be true for both benefits and harms.

Dose response relationship
The relationship between the quantity of treatment given and its effect on outcome.  In meta-analysis, dose-response relationships can be investigated using meta-regression.

Double blind
See blinding.

Dropouts
See attrition