11 Analysis of Survival Data and Data from Matched Studies
Study data may be collected in many di¤erent ways. In addition to surveys,
which are cross-sectional, as shown in many examples in previous chapters,
biomedical research data may come from di¤erent sources, the two fundamen-
tal designs being retrospective and prospective. Retrospective studiesgather
past data from selected cases and controls to determine di¤erences, if any, in
exposure to a suspected risk factor. They are commonly referred to ascase–
control studies; each case–control study is focused on a particular disease. In a
typical case–control study, cases of a specific disease are ascertained as they
arise from population-based registers or lists of hospital admissions, and con-
trols are sampled either as disease-free persons from the population at risk or as
hospitalized patients having a diagnosis other than the one under study. The
advantages of a retrospective study are that it is economical and provides
answers to research questions relatively quickly because the cases are already
available. Major limitations are due to the inaccuracy of the exposure histories
and uncertainty about the appropriateness of the control sample; these prob-
lems sometimes hinder retrospective studies and make them less preferred than
prospective studies.Prospective studies, also calledcohort studies, are epidemi-
ological designs in which one enrolls a group of persons and follows them over
certain periods of time; examples include occupational mortality studies and
clinical trials. The cohort study design focuses on a particular exposure rather
than a particular disease as in case–control studies. Advantages of a longitudi-
nal approach include the opportunity for more accurate measurement of expo-
sure history and a careful examination of the time relationships between expo-
sure and any disease under investigation. An important subset of cohort studies
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