Wait-list controls: Subjects in a clinical trial who receive the intervention, but only after its affect has been assessed in subjects assigned to the experimental group. [Psychosomatic Medicine, 2000, 62, 614–22.]

Wald's test: A test for the hypothesis that a set of related parameters all take the value zero.

Wald―Wolfowitz test: A distribution-free method for testing the null hypothesis that two samples come from identical populations. The test is based on a count of the number of runs. [Hollander, M. andWolfe, D. A., 1999, Nonparametric Statistical Methods, J. Wiley & Sons, New York.]

Ward's method: An agglomerative hierarchical clustering method in which a sum-of-squares criterion is used to decide on which individuals or clusters should be fused at each stage in the procedure. See also single linkage clustering, average linkage clustering, complete linkage cluster analysis and K-means cluster analysis.

Warning lines: Lines on a control chart indicating a mild degree of departure from a desired level of control.

Washout period: An interval introduced between the treatment periods in a crossover design in an effort to eliminate possible carry-over effects. [Senn, S., 1997, Statistical Issues in Drug Development, J.Wiley & Sons, Chichester.]

Weibull distribution: A probability distribution that occurs in the analysis of survival data and has the important property that the corresponding hazard function can be made to increase with time, decrease with time, or remain constant, depending on the values of a particular parameter that characterizes the distribution.