CHP400: Community Health Program - lI Research Methodology STUDY DESIGNS Observational / Analytical Studies Cohort Study Present: Disease Past: Exposure Cross - section Cohort Case - control Present: Disease & Exposure Present: Exposure Future: Disease nbmmb Time is Key

Content Definition Characteristics and types Design Advantages and disadvantages Follow-up Analysis

STUDY DESIGNS Experimental Observational Animal Experiment Human Intervention Clinical trial Analytical Case control Descriptive Case report Case series Cross section Ecological Cohort Research Methodology Cohort Study

A cohort is a group of people who share a common characteristic or experiencecohort Cohort study: An observational, analytic, longitudinal, epidemiologic study in which a particular outcome, such as death from a heart attack, is compared in groups of people who are alike in most ways but differ by a certain characteristic, such as smoking ( EXPOSURE ) STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study Definition:

Characteristics : A “cohort” is a group of people, referred to as “disease-free population” or “population at risk” A survey is first carried out to exclude prevalent cases from the cohort We know the exposure status, looking for the disease status A period of "follow-up“ is specified, for possible new cases' occurrence STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Types: Two types are recognized: Prospective (longitudinal): forward in time follow-up study Retrospective (historical): backward in time study (depends on records: medical / employment). This is the type preferred under occupational settings. STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Population People without disease Exposed Not exposed Disease No disease Disease No disease STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study Design :

YES NO ? ? TIME OUTCOME/DISEASEEXPOSURE COHORT STUDIES CLINICAL TRIALS  STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

YES NO TIME OUTCOME/DISEASEEXPOSURE CASE-CONTROL STUDIES ? ?  STUDY DESIGNS Research Methodology Observational / Analytical Studies

YES NO TIME OUTCOME/DISEASEEXPOSURE YES NO ? ? CASE-CONTROL STUDIES ? ? COHORT STUDIES CLINICAL TRIALS TIME OUTCOME/DISEASEEXPOSURE   STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

INDICATIONS: ?? 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 STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Elements of cohort study Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Selection of study subjects General population Whole population in an area A representative sample Special Exposure groups of population Occupation/professional groups e.g. Radiologists exposure to X-ray STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Personal interviews / mailed questionnaire Reviews of records Dose of drug, radiation, type of surgery etc Medical examination or special test Blood pressure, serum cholesterol Obtaining data on exposure STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

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 Obtaining data on exposure STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

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 Selection of comparison group STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

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 Selection of comparison group STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

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 STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Some loss to follow up is inevitable due to death, change of address, migration, change of occupation. Follow up is the most critical part of the study Follow-up STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Follow-up Techniques : 1.Periodical medical examinations and mailed questionnaires. 2.Direct personal interviews or examinations. 3.Videoconference, neighbors, friends and relatives. STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Follow-up Techniques : 4. Lost persons can be traced through letters, their relatives or friends. 5. Migrated cohort subjects can also be traced through travel and immigration authorities. 6. Dead persons: local or regional Mortality Registers or Death Certificates. STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Problems during Follow-up : Follow-up of a large group. Limited resources. Time constraint. Paucity of trained personnel Attrition STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Attrition Reduction : Obtaining an informed consent. Recording commitment to continue and cooperate in the study. Tracing LOST subjects. Considering Information of lost subjects at the time of analysis Keeping non-response at a low level to improve the validity. STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Calculation of incidence rates among exposed and non exposed groups Estimation of risk Analysis STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

dc ba YesNo Disease Status Yes No Exposure Status a+b b+d a+c Total Study cohort Comparison cohort c+d N Total Incidence among exposed = a a+b Incidence rate Incidence among unexposed = c c+d ANALYSIS STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Estimation of risk ANALYSIS Relative Risk (RR) incidence of disease among exposed Incidence of disease among non-exposed RR= _____________________________ a/a+b c/c+d _________ RR= STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Estimation of risk ANALYSIS Incidence of disease among exposed Incidence of disease among non exposed a/a+b – c/c+d a/a+b AR = = Incidence of disease among exposed _______________________________ _______________ Attributable Risk (AR): STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

SmokingLung cancerTotal SmokingYESNOTotal YES NO Total Calculate RR and AR for above data STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Incidence of lung cancer among smokers 70/7000 = 10 per 1000 Incidence of lung cancer among non-smokers 3/3000 = 1 per 1000 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) STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

The Ideal Cohort : An ideal cohort should be: Stable. Cooperative. Committed Well-informed STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Advantages: No temporal ambiguity (suggests cause- effect relationship) Calculation of incidence rates Suitable for rare exposures Several outcomes can be studied, after follow-up starts. Factors associated with selection cannot influence disease status and hence the results. STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study

Disadvantages : Expensive Time-consuming May be impractical Loss to follow-up may affect sample-size STUDY DESIGNS Research Methodology Observational / Analytical Studies Cohort Study