Observational Studies

Lecture given 9/18/2025

what are the two broad categories that fall under analytical study designs?

observational and experimental 

what two types of studies fall under observational study designs?

cohort study and case-control study

are analytical studies hypothesis generating or hypothesis testing?

hypothesis testing

in observational studies, does the investigator have any control over exposure or disease outcome?

no

what is a cohort?

a group with common characteristics

ex: baby boomers, millenials, class of 2028, ect

cohort studies

longitudinal study over time, where exposure precedes disease outcome

when are cohort studies indicated?

good evidence from previous descriptive studies regarding association of exposure and disease

follow up is relatively easy, cohort is a stable population

what 2 cohorts are typically used?

exposed cohort and unexposed cohort

both cohorts are free of disease and comparable in characteristics other than exposure

what is a famous example of a cohort study?

framingham heart study

longest running, multigenerational longitudinal cohort study

prospective cohort studies

exposure and non-exposure assessed in the present then study group is followed into the future over time to measure disease/outcome

the investigator initiates the study from scratch by enrolling people in a study into the future

retrospective cohort studies

exposure and non-exposure assessed in the past

both exposures and disease outcomes have already occurred by the time the study begins

no disease develops in the future

the study uses data that had been collected for other purposes

what are the pros and cons of prospective cohort studies?

more expensive and time consuming, better information on variables of interest, less efficient to study diseases with long latency periods

what are the pros and cons of retrospective cohort studies?

less expensive and less time consuming, information on variables of interest may be inadequate, efficient to study diseases with long latency periods

fixed cohort

has fixed membership

once group is defined and follow up begins, no one is added

open cohort

aka dynamic cohort

can take on new members during study period and people can leave

what are the advantages of cohort studies?

maintains temporal squence of exposure and disease, can study multiple diseases/outcomes from a given exposure, easy to calculate incidence among exposed and unexposed, minimizes error in exposure assesment, provides complete description of experience subsequent to exposure (including rate of progression and natural history of disease), cohorts sampled from the general population generally have high generalizability

what are the disadvantages of cohort studies?

expensive and time consuming, inefficient in rare diseases, secular trends in technology/behaviors, and changes that may influence behaviors and study characteristics over time thus limiting generalizability

case control study

start with cases and controls and then go back in time to assess their exposure history

how do you select controls in a case control study?

comparability to cases, must be at risk for developing the disease, should resemble cases in all aspects except for the presence of disease, matching of cases and controls

what is the major difference between case control studies and cohort studies?

case control studies cannot calculate incidence rate

what is the most common measure of association for case control studies?

odds ratio

what are the advantages of case control studies?

quick and relatively cheaper, preferred for rare diseases, preferred for diseases with mutlifactorial causes bc can assess multiple exposures

what are the disadvantages of case control studies?

cannot establish ‘cause (exposure) precededs effect (disease outcome)’, cannot estimate incidence rates, not preferred for rare exposures, prone to biases

the hallmark feature of an analytic epidemiologic study is:

A) selecting an appropriate comparison group

B) laboratory confirmation of the diagnosis

C) publication in a peer-reviewed journal

D) complex statistical modeling

A) selecting an appropriate comparison group

a sample of 10,000 women is followed every 5 years for 20 years by researchers who survey lots of their health behaviors and assess incidence of stroke during the study period. which type of study is being used?

A) case control study

B) cohort study

C) ecological study

D) Both B and C

B) cohort study

cohort studies can be:

A) prospective

B) retrospective

C) both

D) neither

C) both