Chapter 12. Observational Study Designs

research class textbook reading-> chapter 12

observational studies in clinical research can be classified as either

analytic or descriptive

analytic observational studies are

similar to randomized, controlled clinical trials

the goal is to estimate the causal effect of an exposure on an outcome

analytical observational studies always include

some type of comparison group

descriptive studies

aim to describe the characteristics or experiences of a particular pt group

well-designed analytic studies can

generate strong evidence for or against a stated hypothesis

analytic v descriptive studies

case-control study

the study population is selected based on a person’s outcome status

cases are

those that have or have had an outcome

controls are those

that lack the outcome

the principle that guides the selection of controls is that they

must be representative of the underlying population that generated the cases

the case-control study study design is well suited to the study of

rare outcomes

challenges of the case-control design

1. identifying a representative sample of controls can be difficult and expensive

2. asking study participants to recall details about past exposure can be problematic

cohort study begins by

classifying exposure status among a group at risk for having the outcome

in a prospective cohort study. . .

both the exposed group and the unexposed group are followed overtime as events occur

in a retrospective cohort study. . .

longitudinal data will have already been collected for the study group→ meaning the events will have already occurred and must be tallied

important characteristics of a well-designed cohort study

1. relative timing of the exposure and the outcome is more certain in a cohort-study than in a case-control study

2. ascertainment of the exposure should be unbiased bc the outcome has not yet occurred when the exposure is measured

3. ascertainment of the outcome should also be unbiased if procedures are in place to collect events systematically for all study subjects→ regardless of exposure status

analytic study designs

case-control studies

cohort studies

descriptive study designs

ecologic studies

cross-sectional (prevalence) studies

case reports and case series

ecologic studies

measurement is performed at the population level rather than at the individual level

the study hypothesis involves individual-level exposures and outcomes

an ecologic study can compare

outcome rates btwn populations in different geographic regions that have different exposures level

OR

it can examine outcome rates within a single population over time to see how they correlate with changes in exposure levels over time

main reason why researchers use ecologic studies

that aggregated rates of disease and other population characteristics are often readily available and can be analyzed quickly and inexpensively

ecologic studies are primarily used for

evidence generation

cross-sectional studies collect data about

health and exposure status of a population at a single point in time

members of a cross-sectional study ________ followed longitudinally

are NOT

in cross-sectional studies

it is possible to compare exposure rates among surveyed individuals with and without a disease, it is not possible to draw conclusions about causation because the timing of the collected information is difficult or impossible to determine

the best cross-sectional studies use

statistical sampling to select study individuals who accurately represent the population of interest

cross-sectional studies are useful for . . .

estimating the disease burden in a population

case report

a written presentation of a particular medical case encountered by a physician

case series

an aggregation of case reports

results of published case reports and case series are

suggestive