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Enrollees in a study who cease participation for one reason or another can create problems for those who analyse and interpret the data. These problems include loss of statistical power, bias in the study results, and lack of generalisability of the study findings. Whether the study is a clinical trial or an observational study these problems may exist. In trials, for example, non-random withdrawals may compromise the comparability between the treatment groups and hence introduce a bias and in observational studies selective withdrawal may limit the generalisability of the study results. Much has been written about the consequences of non-random withdrawals and some authors have offered statistical techniques that can effectively adjust for the resulting bias if certain assumptions are met. Although it is not generally possible to compensate for all of the problems created by attrition, certain considerations like statistical power and adequate representation of important subgroups can …
Funding: supported by contract numbers N01-HR7–8029, 6030, 6031, 6032, 6033, 6034, and 6035 from the National Heart, Lung, and Blood Institute and the National Institute of Allergy and Infectious Diseases and by grant award numbers UO1-HL48534–01, UO1-HL48511–01, UO1-HL48500–01, UO1-HL48518–01, UO1-HL48779–01, UO1-HL48501–01, and UO1-HL48516–01.
Conflicts of interest: none.