Cohort and case-controls studiesPresentation Transcript
COHORT VS. CASE-CONTROL STUDIESAdriana Pérez Fortis
OVERVIEW What is a cohort study? Types of cohort studies Advantages / Disadvantages What to look in cohort studies? What is a case-control study? Advantages / Disadvantages What to watch out for in case- control studies?
What is a cohort study? A cohort study tracks two or more groups forward from exposure to outcome.
What is a cohort study?Compares the experience of a group exposed to somefactor with another group not exposed to the factor.If the former group has a higher or lower frequency of anoutcome than the unexposed, then an associationbetween exposure and outcome is evident.
Types of cohort studies Prospective cohort design Allows exposure to risk factors to be assessed directly and confounding variables to be considered. Retrospective cohort design Is effective for diseases with a long development time. Sometimes referred to as historical cohort studies, they offer the advantage of speed and low cost compared to a prospective cohort.
Cohort studies Advantages Disadvantages The best way to ascertain both Selection bias is built into cohortthe incidence and natural history studies.of a disorder. Is not optimum for rare diseases Are useful in investigation of or those that take a long time tomultiple outcomes that might develop.arise after a single exposure. Loss to follow-up can be a Are also useful in the study of difficulty. Differential losses torare exposures. follow-up between those exposed and unexposed can bias results. Reduce the risk of survivor bias. Can be expensive and time Allow calculation of incidence consuming.rates, relative risks, andconfidence intervals.
What to look in cohort studies?a. Who is at risk? How much selection bias was present?b. Who is exposed? What steps were taken toc. Who is an appropriate minimize information bias? control? How complete was thed. Have outcomes been follow-up of both groups? assessed equally? Were potential confoundinge. Have losses been factors sought and minimized? controlled for in the analysis?
What is a case-control study? Is an analytical observation study, which has a comparison (control) group. Case control studies are retrospective, and the main objective is to determine whether or not an association exists between a disease and a particular risk factor.
What is a case-control study? They should not be confused with historical cohort studies (also retrospective). Case control studies trace backwards from outcome to exposure. Cohort studies: Study Case-control studies: groups are defined by Study groups are defined exposure. by outcome.
Case-control studies Advantages Disadvantages Is the most efficient design in If the frequency of exposure isterms of time, money, and effort. low, case-control studies quickly become inefficient. Are also efficient in theinvestigation of diseases that have Many methodological issuesa long latency period. affect the validity of the results of case-control studies. Are useful to study rarediseases. Are prone to selection and recall bias. Can study multiple riskfactors/exposures. It can be difficult to choose an appropriate control group. In general cohort studies can be more efficient than case-control studies.
What to watch out for in case-control studies?Selection of case and control groups. Researchers should detail eligibility criteria used for selection. Controls should represent the population at risk of becoming cases. Selection of controls must be independent of the exposure being investigated.
What to watch out for in case-control studies?Measurement of exposure information. Participants, might inaccurately remember past exposures, especially those that happened a long time ago. This differential recall (recall bias) causes information bias. Differential recall between cases and controls led to a biased estimate of risk. Investigators who do case-control studies must be aware of the potential for information bias. Reports of case-control studies that do not detail use of memory aids, should make readers skeptical.
What to watch out for in case-control studies?Control for confounding. Case-control studies need to address confounding bias. Invalid measurement of potential confounding factors leads to residual confounding, even after adjustment.
REFERENCESGrimes, D. A., & Schulz, K. F. (2002). Cohort studies: marchingtowards outcomes. The Lancet 359, 341-345.Schulz, K. F., & Grimes, D. A. (2002). Case-control studies:research in reverse. The Lancet 359, 431-434.