say that he or she has obtained patients as a random sample from a parent
population of patients.
12.1 TYPES OF STUDY DESIGNS
In addition to surveys that are cross-sectional, as seen in many examples in
earlier chapters, study data may be collected in many di¤erent ways. For
example, investigators are faced more and more frequently with the problem of
determining whether a specific factor or exposure is related to a certain aspect
of health. Does air polution cause lung cancer? Do birth control pills cause
thromboembolic death? There are reasons for believing that the answer to each
of these and other questions is yes, but all are controversial; otherwise, no
studies are needed. Generally, biomedical research data may come from di¤er-
ent sources, the two fundamental designs being retrospective and prospective.
But strategies can be divided further into four di¤erent types:
- Retrospective studies (of past events)
- Prospective studies (of past events)
- Cohort studies (of ongoing or future events)
- Clinical trials
Retrospective studiesof past events gather past data from selected cases,
persons are identified who have experienced the event in question, and controls,
persons who have not experienced the event in question, to determine di¤er-
ences, if any, in exposure to a suspected risk factor under investigation. 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 spe-
cific disease are ascertained as they arise from population-based registers or
lists of hospital admissions, and controls are sampled either as disease-free
persons from the population at risk or as hospitalized patients having a diag-
nosis other than the one under study. An example is the study of thromboem-
bolic death and birth control drugs. Thromboembolic deaths were identified
from death certificates, and exposure to the pill was traced by interview with
each woman’s physician and a check of her various medical records. Control
women were women in the same age range under the care of the same physi-
cians.
Prospective studiesof past events are less popular because they depend on
the existence of records of high quality. In these, samples of exposed subjects
and unexposed subjects are identified in the records. Then the records of the
persons selected are traced to determine if they have ever experienced the event
to the present time. Events in question are past events, but the method is called
prospectivebecause it proceeds from exposure forward to the event.
Cohort studiesare epidemiological designs in which one enrolls a group of
persons and follows them over certain periods of time; examples include occu-
446 STUDY DESIGNS