644 Obstetrics and Gynecology Board Review •••
❍ What are confounders?
Confounding relationships are relationships where the factor of interest is related to another factor that is also
influencing the outcome of interest.
❍ What are effect modifiers (or interactions)?
An effect modifier (or interaction) is a third variable that alters the association between an exposure and a disease
outcome. With an effect modifier, the exposure may not have the same effect in all settings or subgroups.
❍ What is age adjustment?
Age adjustment takes summary rates for various populations and removes differences in age distributions so that
the adjusted rate is independent of age. Age adjustment is used when the age profiles of the populations being
compared are quite different.
❍ What is a cohort study?
A cohort study is an observational study that tracks health information over a period of time. It provides
information on disease development and allows study of the natural history of a disease. Cohort studies are good
for studying common diseases where exposure and risk factors result in measurable disease outcome in a reasonable
amount of time following exposure.
❍ What is meant by the phrase, “natural history” of an illness?
Natural history of an illness is the progression of a disease through successful stages, often used to describe the
course of an illness for which no effective treatment is available.
❍ Define case fatality.
It is a way of characterizing the natural history of an illness. It is represented as the percentage of patients with a
disease who die within a specified observation period. Case fatalities are estimated in the following way:
CF = D/A where CF is case fatality, D is the number of deaths, and A is the number of diagnosed patients. The
resulting estimate can be left as a proportion or multiplied by 100 to convert it to a percentage.
❍ What is a cross-sectional study?
Cross-sectional studies are observational studies that collect descriptive information about subjects; all subjects
are measured at the same point in time. It is a relatively simple and inexpensive study design because there is no
follow-up involved.
❍ What is a case-control study?
A case-control study is an observational study in which subjects are sampled based on the presence (case) or
absence (controls) of a disease of interest. Information is collected about earlier exposure to risk factors of interest.
❍ What is a RCT?
A RCT is an experimental study in which subjects are randomly assigned to treatment groups. It is the most
robust design for investigating a causal relationship between a treatment and its effect. Randomization overcomes
selection bias and ensures confounders are equally distributed among groups.