ZELEN DESIGN (Syn: prerandomization design, post-randomized consent) A modi- fied double-blind randomized controlled trial design proposed by Marvin Zelen (b. 1927).392 The essential feature of the Zelen design is randomization before informed consent procedures, which are claimed to be needed only for the group allocated to receive the experimental regimen. Many ethicists disagree, holding that it is necessary to obtain the informed consent of all participants regardless of the group to which they are allocated.

ZERO POPULATION GROWTH The status of a population in which there is no net increase of numbers; the number of births (plus immigrants) equals the number of deaths (plus emigrants).

ZERO REPORTING Reporting of the absence of cases of a disease under surveillance; this ensures that participants have not merely forgotten to report.

ZERO-SUM GAME A situation in which one participant can “gain” only at the expense of or to the detriment of another.

ZERO-TIME SHIFT The selection of a starting point for the measurement of survival following the detection of disease. It denotes the movement “backward” (toward the start- ing point of a disease) of time between onset and detection that may accompany use of a screening procedure.

ZOONOSIS An infection or infectious disease transmissible under natural conditions from vertebrate animals to humans. Examples include rabies and plague. May be enzo- otic or epizootic.

Z SCORE Score expressed as a deviation from the mean value in standard deviation units; the term is used in analyzing continuous variables, such as heights and weights, of a sample, to express results of behavioral tests, etc. The use of Z scores has been criticized for reducing interpretability and comparability of results from different studies or populations.