Study Designs A. A case-control study is a study in which the frequency of past exposure is compared between one group of individuals who do not have the outcome of interest (controls) and another group of individuals that do have the outcome of interest (cases). B. A cohort study involves following a chosen group of people over a period of time, and later comparing the incidence of disease between individuals who were exposed and those who were not exposed. C. A cross-sectional study is one that assesses the exposure and outcome at the same point in time.
D. A clinical trial is a highly controlled study that evaluates the efficacy and safety of medications or medical devices. Measure of Occurrence E. Incidence Measures i. Incidence rate – The frequency with which susceptible individuals transition from being at risk to being diseased. ii. Hazard rate – An individual’s probability of becoming a case at a precise time, given that they are at risk at that time. iii. Cumulative incidence – The total number of new cases in the population at risk and within a specific period of time
F. Prevalence Measures i. Point prevalence – The number of people in a population that are diseased at a specific point in time. ii. Period prevalence – The number of people in a population who become cases over a certain period of time. Measure of Association G. Relative Measures i. Risk ratio – The probability of an individual becoming a case among a group which is exposed compared to the probability of an individual becoming a case among a group which is not exposed. ii.
Rate ratio – The incidence rate in the exposed group compared to the incidence rate in the unexposed group. iii. Odds ratio – The odds that individuals who are cases were exposed compared to the odds that individuals who are non-cases were exposed. iv. Hazard ratio – The ratio of the hazard rates in the exposed group compared to the unexposed group. H. Absolute Measures i. Risk/rate/incidence difference – The difference between the observed risk/rate/incidence of the outcome of interest in the exposed and unexposed groups. i. Attributable risk – The difference in the rate of an outcome of interest between the exposed group and the unexposed group. Other Measures I. Sensitivity – The proportion of true cases that are identified as such by a test, compared to a gold standard. [True Positive/(True Positive + False Negative)] J. Specificity – The proportion of true non-cases that are identified as such by a test, compared to a gold standard. [True Negative/(True Negative + False Positive)]
