Zelen's single-consent design: An alternative to simple random allocation for forming treatment groups in a clinical trial. Begins with the set of N eligible patients. All N of these patients are then subdivided randomly into two groups, say groups G1 and G2of sizes n1 and n2. The standard therapy is applied to all the patients assigned toG1. The new therapy is assigned only to those patients in G2 who consent to its use. The remaining patients who refuse the new treatment are treated with the standard therapy. The main advantages of the design are that almost all eligible individuals are included in the trial and that it allows the evaluation of the true effect of offering experimental interventions to patients. The main disadvantages are that such trials have to be open-label trials and that the statistical power of the study may be affected as a high proportion of participants choose to have the standard treatment. [New England Journal of Medicine, 1979, 300, 12425]

Zero-sum game: A game played by a number of people in which the winner takes all the stakes provided by the losers so that the algebraic sum of gains at any stage is zero. Many decision problems can be modelled as such games involving two people. [New Scientist, 1990, 4, 1.]

Z-scores: Synonym for standard scores.

z-test: A test for assessing the hypothesis that the mean of a normal distribution takes a particular value, or for assessing whether the means of two normal distributes with the some variance are equal.