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COHORT STUDY Study Design:

COHORT STUDY Study Design Dr. Nitin V Solanki (M.D.) Assistant Professor Department of Community Medicine, Gujarat Adani Institute Of Medical Sciences, Bhuj


Epidemiology Defined by John M. Last in 1988 “Study of Distribution and Determinants of health related state or event in a specified population and the application of this study to the control of health problem”. We measure – Disease frequency Diseases distribution Determinants of disease.

Descriptive Epidemiology:

Descriptive Epidemiology Describe the disease by Time Place Person

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Hierarchy of Epidemiologic Study Design

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Cohort study is undertaken to support the existence of association between suspected cause and disease A major limitation of cross-sectional surveys and case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome . Cohort Study: Key Point: Presence or absence of risk factor is determined before outcome occurs.



Elements that may define a study population for a cohort:

Elements that may define a study population for a cohort Residence Demographic characteristics Cultural background Socio-economic group Employment Sharing a common experience or condition Population

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Cohort studies longitudinal Prospective studies Forward looking study Incidence study Panel study starts with people free of disease assesses exposure at “baseline” assesses disease status at “follow-up”

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Cohort study Past Present Future Patient population Not exposed Evaluate for outcome Outcome Exposed No outcome Appropriate patient spectrum Outcome No outcome


INDICATION OF A COHORT STUDY When there is good evidence of exposure and disease. When exposure is rare but incidence of disease is higher among exposed When follow-up is easy, cohort is stable When ample funds are available

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b+d Frame work of Cohort studies c c+d a a+b Total Yes Disease Status Yes No Exposure Status b d a+c N No Study cohort Comparison cohort

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General consideration while selection of cohorts Both the cohorts are free of the disease. Both the groups should equally susceptible to disease Both the groups should be comparable Diagnostic and eligibility criteria for the disease should be defined well in advance .

Elements of cohort study :

Elements of cohort study Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis

Selection of study subjects:

Selection of study subjects General population Whole population in an area A representative sample Special group of population Select group occupation group / professional group (Dolls study ) Exposure groups Person having exposure to some physical, chemical or biological agent e.g. X-ray exposure to radiologists

Obtaining data on exposure:

Obtaining data on exposure Personal interviews / mailed questionnaire Reviews of records Dose of drug, radiation, type of surgery etc Medical examination or special test Blood pressure, serum cholesterol Environmental survey By obtaining the data of exposure we can classify cohorts as Exposed and non exposed and By degree exposure we can sub classify cohorts

Selection of comparison group:

Selection of comparison group Internal comparison Only one cohort involved in study Sub classified and internal comparison done External comparison More than one cohort in the study for the purpose of comparison e.g. Cohort of radiologist compared with ophthalmologists Comparison with general population rates If no comparison group is available we can compare the rates of study cohort with general population. Cancer rate of uranium miners with cancer in general population

Follow-up :

Follow-up To obtain data about outcome to be determined (morbidity or death) Mailed questionnaire, telephone calls, personal interviews Periodic medical examination Reviewing records Surveillance of death records Follow up is the most critical part of the study Some loss to follow up is inevitable due to death change of address, migration, change of occupation. Loss to follow-up is one of the draw-back of the cohort study.

Incidence rates of outcome:

Incidence rates of outcome N d c b a Yes No Disease Status Yes No Exposure Status a+b c+d b+d a+c Total Study cohort Comparison cohort


Analysis Incidence rates among exposed and not exposed Estimation of risk Relative risk Attributable risk Population attributable risk

Incidence rate:

Incidence rate Incidence among exposed = a a+b Incidence among non-exposed = c c+d

Relative Risk (RR):

Relative Risk (RR) It is the “ratio of incidence of disease among exposed to incidence of disease among non- exposed”. Incidence among exposed Relative Risk = ---------------------------------- Incidence among not exposed

Relative Risk Contd.:

Relative Risk Contd. Also called as “ Risk Ratio” It is a direct measure of “ strength of association” between the cause and effect RR=1- No association RR=2- the disease is 2 times higher in the exposed to that of non exposed RR= 0.25 - 75% reduction in the incidence rates in the exposed to that of not exposed

Attributable Risk (AR) :

Attributable Risk (AR) It is the “difference in the incidence rate among exposed to that of non exposed”. Also known as “Risk difference”. incidence among exposed – incidence among non-exposed AR = ------------------------------------------------ incidence rate among exposed


ATTRIBUTABLE RISK Contd. AR indicates to what extent the disease under study can be attributed to the exposure. This suggests that the amount of disease that might be eliminated if the factor under study could be controlled or eliminated.

Population Attributable Risk:

Population Attributable Risk It is the “incidence of disease in the total population minus the incidence of disease among those who are not exposed to the suspected causal factor”. It shows that how much disease could be eliminated in the population by eliminating the suspected causal factor.

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Smoking Lung cancer Total YES NO YES 70 6930 7000 NO 3 2997 3000 73 9927 10000 Find out RR and AR for above data

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Incidence of lung cancer among smokers 70/7000 = 10 per 1000 Incidence of lung cancer among non-smokers =3/3000 = 1 per 1000

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Relative Risk(RR) = 10 / 1 = 10 (lung cancer is 10 times more common among smokers than non smokers)

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Attributable risk(AR) = 10 – 1/10 X 100 = 90 % (90% of the cases of lung cancer among smokers are attributed to their habit of smoking)

Types of Cohort Study:

Types of Cohort Study Prospective cohort study Retrospective (historical) cohort study Combination of Retrospective and Prospective cohort study.

Prospective cohort study:

time Exposure Study starts Disease occurrence Prospective cohort study time Exposure Study starts Disease occurrence EPIET ( www )

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Retrospective cohort studies Exposure time Disease occurrence Study starts EPIET ( www )

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Cohort Studies Grimes & Schulz, 2002 ( www ) ( PDF )

Advantages of cohort study :

Advantages of cohort study Incidence can be calculated Cohort studies offer the possibility of studying the full range of effects of the suspected etiological factor like natural H/o disease and other possible related health conditions which may not be suspected at the beginning of the study. Several possible outcomes can be studied Cohort studies provide relative risk Dose response can also be calculated Since the comparison groups are formed before the disease develops bias can be eliminated Cohort analysis is that the study design does not require strict random assignment of subjects

Disadvantages of cohort study:

Disadvantages of cohort study Selection of comparison groups may be difficult. Those who volunteer may not be representing the population Changes in diagnostic criteria over time Expensive Study may alter peoples behavior Practical problems dictate that we should concentrate on limited factors Ethical problems

Differences between Case control and Cohort studies:

Differences between Case control and Cohort studies Case- Control Study Cohort Study proceeds from effect to cause Proceeds from cause to effect Starts with the disease Starts with the people exposed to particular cause Tests whether the suspected cause occurs more frequently with the disease or without the disease Tests whether the disease occurs more frequently in those exposed than in not exposed Usually first approach in testing a hypothesis Reserved for previously formulated hypothesis

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Case- Control Study Cohort Study Involves fewer no. of subjects Large no. of subjects. Quick results Long follow up with delayed results Suitable for rare diseases Inappropriate when the disease is rare Yields only RR (Odds ratio) Both RR and AR Cannot yield information about diseases other than the one selected for the study. Can yield Information about more than one outcome. Relatively inexpensive Expensive.

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