cohort study

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COHORT STUDY:

COHORT STUDY DR. A.A.TRIVEDI (M.D., D.I.H.) ASSISTANT PROFESSOR email : dr_aatrivedi@yahoo.com

Epidemiology:

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.

TYPES OF EPIDEMIOLOGICAL STUDIES:

TYPES OF EPIDEMIOLOGICAL STUDIES 1. OBSERVATIONAL STUDIES A. DESCRIPTIVE STUDY DESCRIBE DIESEASE BY TIME PLACE PERSON B. ANALYTICAL STUDIES ECOLOGICAL STUDY CROSS SECTIONAL STUDY CASE-CONTROL STUDY COHORT STUDY 2. EXPEREMENTAL STUDIES RANDOMIZED CONTROLLED TRIAL (RCT) FIELD TRIAL COMMUNITY TRIAL

Descriptive Epidemiology:

Descriptive Epidemiology Describe the disease by Time Place Person

Cohort study is undertaken to support the existence of association between suspected cause and disease:

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.

WHAT IS COHORT:

WHAT IS COHORT Ancient Roman military unit, A band of warriors. Persons banded together. Group of persons with a common statistical characteristic. [Latin] E.g. age, birth date,

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

INDICATION OF A COHORT STUDY:

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.

ANALYSIS:

ANALYSIS Calculation of incidence rates among exposed and non exposed groups Estimation of risk

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

Incidence rate:

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

Estimation of risk:

Estimation of risk Relative Risk incidence of disease among exposed RR = ______________________________ Incidence of disease among non-exposed a/a+b = _________ c/c+d

Estimation of Risk:

Estimation of Risk Attributable Risk Incidence of disease among exposed – incidence of disease among non exposed AR = _______________________________ Incidence of disease among exposed a/a+b – c/c+d AR = _______________ a/a+b

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

Cohort studies:

Cohort studies Strengths We can find out incidence rate and risk More than one disease related to single exposure can establish cause - effect good when exposure is rare minimizes selection and information bias Weaknesses losses to follow-up often requires large sample ineffective for rare diseases long time to complete expensive Ethical issues

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THANK YOU

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