The main objective of this session will be to present a brief comparison between both designs, highlighting the main aspects that could threat the validity of the designs.
The term cohort has military, not medical, roots. A cohort was a 300–600-man unit in the Roman army; ten cohorts formed a legion. The etymology of the term provides a useful mnemonic: a cohort study consists of bands or groups of persons marching forward in time from an exposure to one or more outcomes.
In its simplest form, a cohort study compares the experience of a group exposed to some factor with another group not exposed to the factor.
Researchers doing this kind of study must, therefore, go forward in time from the present or go back in time to choose their cohorts. Either way, a cohort study moves in the same direction, although gathering data might not. Historical cohort studies should not be confused with case-control studies (also retrospective). Cohorts track people forward in time from exposure to outcome. Case control studies trace backwards from outcome to exposure.
All participants (both exposed and unexposed) in a cohort study must be at risk of developing the outcome. Cohort studies need a clear, unambiguous definition of the exposure at the outset. The key notion is that controls (the unexposed) should be similar to the exposed in all important respects, except for the lack of exposure. Outcomes must be defined in advance; they should be clear, specific, and measurable. Identification of outcomes should be comparable in every way for the exposed and unexposed to avoid information bias. Reduction of loss to follow-up over time is a challenge, since differential losses to follow-up introduce bias.
SOLVING THE DIET-CANCER MISTERY: SCIENTIFIC STUDIES PROVIDE CLUES. Cohort studies gather data on a large group of healthy people and then follow that group over many years. Study participants may keep daily food diaries or fill out questionnaires about what they eat. As some people in the study develop cancer, researchers zero in on how their diets differed from the people who remained healthy. Did the people who stayed cancer-free eat more blueberries or soy products than the people who developed cancer? What they ate might provide a clue.
Starting with an outcome like disease, these studies look backwards in time for exposures that might have caused the outcome.
Investigators should state how the sample was selected, providing a clear definition of the outcome being studied. Researchers should detail eligibility criteria used for selection. They should gather data preferably from incident rather than prevalent cases. Controls should represent the population at risk of becoming cases. Selection of controls must be independent of the exposure being investigated. If investigators do not select control groups independent of exposure, biases in either direction might result.
Cohort and case-controls studies
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.