Types of Observational Studies
Observational studies are of three basic types:
A sample survey is a study that provides information about a population at a
particular point in time (current information).
A prospective study is a study that observes a population in the present using a
sample survey and proceeds to follow the subjects in the sample forward in time
in order to record the occurrence of specific outcomes.
A retrospective study is a study that observes a population in the present using a
sample survey and also collects information about the subjects in the sample
regarding the occurrence of specific outcomes that have already taken place.
Example of Sample survey Opinion polls are constantly in the news, and the
names of Gallup and Harris have become well known to everyone. These polls, or
sample surveys, reflect the attitudes and opinions of citizens on everything from
politics and religion to sports and entertainment.
Example of a prospective study We could identify a group of disease free subjects
and then follow them over a period time of time until some of the individuals
develop the disease. The development or nondevelopment of the disease would
then be related to other variables measured on the subjects at the beginning of
the study.
Example of a prospective study A study was conducted to determine if women
taking oral contraceptives had a greater propensity to develop heart disease. A
group of 5000 women currently using oral contraceptives and another group of
5000 women not using oral contraceptives were selected for the study. At the
beginning of the study, all 10,000 women were given physicals and were found to
have healthy hearts. The women’s health was then tracked for a 3-year period. At
the end of the study, 15 of the 5,000 users had developed a heart disease,
whereas only 3 of the nonusers had any evidence of heart disease.
Example of retrospective study A study identifies two groups of subjects: 1)
subjects with the disease and 2) subjects without the disease. The researcher
then attempts to see if the subjects prior health habits are related to their current
health status.
Example of retrospective study A study was designed to determine if people who
use public transportation to travel to work are more politically active than people
who use their own vehicle to travel to work. A sample of 100 people in a large
urban city was selected from each group, and then all 200 individuals were
interviewed concerning their political activities over the past 2 years. Out of the
100 people who used public transportation, 18 reported that they had actively
assisted a candidate in the past 2 years, whereas only 9 of the 100 persons who
used their own vehicles stated that they had participated in a political campaign.