RESEARCH SERIES Observational research methods. …

1 RESEARCH SERIESO bservational RESEARCH methods . RESEARCH design II:cohort, cross sectional, and case-control studiesC J Med J2003;20:54 60 Cohort, cross sectional, and case-control studies arecollectively referred to as Observational studies. Oftenthese studies are the only practicable method ofstudying various problems, for example, studies ofaetiology, instances where a randomised controlled trialmight be unethical, or if the condition to be studied israre. Cohort studies are used to study incidence, causes,and prognosis. Because they measure events inchronological order they can be used to distinguishbetween cause and effect.

2 Cross sectional studies areused to determine prevalence. They are relatively quickand easy but do not permit distinction between causeand effect. Case controlled studies compare groupsretrospectively. They seek to identify possible predictorsof outcome and are useful for studying rare diseases oroutcomes. They are often used to generate hypothesesthat can then be studied via prospective cohort or , cross sectional, and case-controlstudies are often referred to as observa-tional studies because the investigator sim-ply observes. No interventions are carried out bythe investigator.

3 With the recent emphasis on evi-dence based medicine and the formation of theCochrane Database of randomised controlledtrials, such studies have been somewhat gliblymaligned. However, they remain important be-cause many questions can be efficiently answeredby these methods and sometimes they are theonly methods objective of most clinical studies is todetermine one of the following prevalence, inci-dence, cause, prognosis, or effect of treatment; itis therefore useful to remember which type ofstudy is most commonly associated with eachobjective (table 1)While an appropriate choice of study design isvital, it is not sufficient.

4 The hallmark of goodresearch is the rigor with which it is conducted. Achecklist of the key points in any study irrespec-tive of the basic design is given in box published study should contain suffi-cient information to allow the reader to analysethe data with reference to these key this article each of the three importantobservational RESEARCH methods will be discussedwith emphasis on their strengths and weak-nesses. In so doing it should become apparentwhy a given study used a particular researchmethod and which method might best answer aparticular clinical STUDIEST hese are the best method for determining theincidence and natural history of a condition.

5 Thestudies may be prospective or retrospective andsometimes two cohorts are cohort studiesA group of people is chosen who do not have theoutcome of interest (for example, myocardial inf-arction). The investigator then measures a varietyof variables that might be relevant to the develop-ment of the condition. Over a period of time thepeople in the sample are observed to see whetherthey develop the outcome of interest (that is,myocardial infarction).In single cohort studies those people who donot develop the outcome of interest are used asinternal two cohorts are used, one group hasbeen exposed to or treated with the agent ofinterest and the other has not, thereby acting asan external cohort studiesThese use data already collected for otherpurposes.

6 The methodology is the same but thestudy is performed posthoc. The cohort is followed up retrospectively. The study periodmay be many years but the time to complete thestudy is only as long as it takes to collate and ana-lyse the and disadvantagesThe use of cohorts is often mandatory as arandomised controlled trial may be unethical; forexample, you cannot deliberately expose people tocigarette smoke or asbestos. Thus RESEARCH on riskfactors relies heavily on cohort cohort studies measure potential causesbefore the outcome has occurred the study candemonstrate that these causes preceded theTable 1 ObjectiveCommon designPrevalenceCross sectionalIncidenceCohortCause (in order ofreliability)Cohort, case-control, crosssectionalPrognosisCohortTreatment effectControlled ,Departmentof Accident and EmergencyMedicine, Taunton andSomerset Hospital,Taunton, Somerset, UKCorrespondence to:Dr C J on July 2, 2022 by guest.

7 Protected by Med J: first published as on 1 January 2003. Downloaded from outcome, thereby avoiding the debate as to which is cause andwhich is further advantage is that a single study can examinevarious outcome variables. For example, cohort studies ofsmokers can simultaneously look at deaths from lung, cardio-vascular, and cerebrovascular disease. This contrasts withcase-control studies as they assess only one outcome variable(that is, whatever outcome the cases have entered the studywith).Cohorts permit calculation of the effect of each variable onthe probability of developing the outcome of interest (relativerisk).

8 However, where a certain outcome is rare then aprospective cohort study is inefficient. For example, studying100 A&E attenders with minor injuries for the outcome ofdiabetes mellitus will probably produce only one patient withthe outcome of interest. The efficiency of a prospective cohortstudy increases as the incidence of any particular outcomeincreases. Thus a study of patients with a diagnosis of deliber-ate self harm in the 12 months after initial presentation wouldbe efficiently studied using a cohort problem with prospective cohort studies is the lossof some subjects to follow up.

9 This can significantly affect theoutcome. Taking incidence analysis as an example (incidence= cases/per period of time), it can be seen that the loss of a fewcases will seriously affect the numerator and hence the calcu-lated incidence. The rarer the condition the more significantthis studies are much cheaper as the data havealready been collected. One advantage of such a study designis the lack of bias because the outcome of current interest wasnot the original reason for the data to be collected. However,because the cohort was originally constructed for anotherpurpose it is unlikely that all the relevant information willhave been rigorously cohorts also suffer the disadvantage thatpeople with the outcome of interest are more likely to remem-ber certain antecedents, or exaggerate or minimise what theynow consider to be risk factors (recall bias).

10 Where two cohorts are compared one will have beenexposed to the agent of interest and one will not. The majordisadvantage is the inability to control for all other factors thatmight differ between the two groups. These factors are knownas confounding confounding variable is independently associated withboth the variable of interest and the outcome of interest. Forexample, lung cancer (outcome) is less common in peoplewith asthma (variable). However, it is unlikely that asthma initself confers any protection against lung cancer. It is moreprobable that the incidence of lung cancer is lower in peoplewith asthma because fewer asthmatics smoke cigarettes (con-founding variable).