WASHOUT PHASE That stage in a study, especially a therapeutic trial, when symptom- atic treatment is withdrawn so that its effects disappear and the subject’s characteristics return to their baseline state.14,330 An effective washout phase is essential in a crossover experiment.

WEB OF CAUSATION (Syn: causal web). In epidemiology and public health, a popular metaphor for the theory of sequential and linked multiple causes of diseases and other health states (multiple cause theory). The term appears in several monographs on epidemiology published in the early 1960s169 and was probably first published in 1959 by Dawber et al.390 and in 1960 by Brian MacMahon (1923–2007) et al.391 Originally deployed mainly for an epidemiology practised at the individual level of organization (although not necessarily confined to it), the metaphor can be extended to incorporate a sequence of multiple dimensions.89 See also association; causal diagrams; diseases of complex etiology; ecoepidemiology; multilevel analysis; multiple causation; probability of causation.

WEIGHTED AVERAGE A value determined by assigning weights to individual mea- surements or estimates. Each value is assigned a nonnegative coefficient (weight); the sum of the products of each value by its weight divided by the sum of the weights is the weighted average.

WEIGHTED SAMPLE A sample that is not strictly proportional to the distribution of classes in the universe population. A weighted sample has been adjusted to include larger proportions of some than other parts of the universe population because those parts accorded greater “weight” would otherwise not have sufficient numbers in the sample to lead to generalizable conclusions or because they are considered to be more important, more interesting, more worthy of detailed study, etc.

WER Weekly Epidemiological Record An instrument for the rapid and accurate dissemi- nation of epidemiological information on cases and outbreaks of diseases under the International Health Regulations and on other communicable diseases of public health importance, including emerging or reemerging infections. Published by the WHO (www.who.int/wer).


WHA World Health Assembly. The supreme decision-making body for the World Health Organization. It meets once a year and is attended by delegations from all of WHO’s over 190 member states.

“WHISTLE-BLOWING” Informing authorities or the media when fraud or misrepresen- tation of research results or any other form of wrongdoing is suspected.

WHO World Health Organization (www.who.int).

WILD TYPE The normal, nonaltered sequence of a gene; the opposite is a mutated sequence.

WONCA World Organization of National Colleges, Academies, and Academic Associa- tions of General Practitioners/Family Physicians, the World Organization of Family Doctors.257 See also International Classification of Primary Care, Second Edition Revised (ICPC-2-R).

WOOLF-HALDANE CORRECTION A modification of the observed data that permits statistical analysis when cell(s) in a table have a value of zero. An increment close to zero (e.g., 0.5) is added to all the cells to enable computation of the stratum-specific odds ratio and other quantities.97 Now largely obsolete owing to the availability of pack- aged software for methods that require no such correction (e.g., Fisher’s exact test and the Mantel-Haenszel odds ratio).

WORKUP BIAS Bias due to incorrectly or incompletely diagnosed cases being more numerous in one group than another in a study in which comparison is made between groups. Usually this happens because patients with a positive screening-level test receive a more thorough workup with diagnostic (“gold standard”) tests than those whose screening-level test was negative.63,327,369 It is a form of verification bias. See also spectrum bias.

WORM COUNT A method of surveillance of helminth infection of the gut that depends upon counts of worms, or cysts or ova, in quantitatively titrated samples of feces. Other terms to describe this form of surveillance are egg count, cyst count, and parasite count.