The most common type of retrospective observational study is a case
series, which presents information from multiple patients
with a similar disease or condition.
The data usually come from a review of charts, looking back in time
to what occurred with each patient.
A case series can generate speculative associations, which
can be further tested by other study types.
The major drawback is a lack of any control group with which to
compare results.
An improvement on case series is the case-control study, a
case series with a historical control
Because many case-control studies have relatively small numbers of
patients, it is useful to choose control patients matched by gender,
age, pathology, and/or other experimental variables.