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01/09/2015COMMUNITY MEDICINE SEMINAR
1
STEPS IN COHORT
STUDY
CONTENTS
01/09/2015COMMUNITY MEDICINE SEMINAR
2
 Introduction
 What is a cohort?
 Elements of a cohort study
 Selection of study subjects
 Obtaining data on exposure
 Selection of comparison groups
 Follow up
 Analysis
 References
INTRODUCTION
01/09/2015COMMUNITY MEDICINE SEMINAR
3
 Cohort study is a type of analytical study
 Done to support or refute the existence of an
association between suspected cause and disease
 Prospective / longitudinal /incidence / forward
looking study
 CAUSE → EFFECT
 Types-Prospective, Retrospective, Combination
01/09/2015COMMUNITY MEDICINE SEMINAR 4
WHAT IS A COHORT ???
01/09/2015COMMUNITY MEDICINE SEMINAR
5
 COHORT-a group of people who share a common
characteristic or experience within a defined time
period.
 STUDY AND CONTROL COHORTS
 Both should be free of disease, equally susceptible to
the disease, and both groups should be comparable
to each other.
FRAMEWORK OF COHORT STUDY
01/09/2015COMMUNITY MEDICINE SEMINAR
6
ELEMENTS OF A COHORT
01/09/2015COMMUNITY MEDICINE SEMINAR
7
1.Selection of study subjects
2.Obtaining data on exposure
3.Selection of a comparison group
4.Follow up
5.Analysis
SELECTION OF STUDY SUBJECTS
01/09/2015COMMUNITY MEDICINE SEMINAR
8
• General population- If exposure is frequent, an
appropriate sample size(exposed and unexposed) is
taken from general population.
• Special groups -
- like selected profession, college alumini,
antenatal women, etc.
-exposure groups-if exposure is rare, select a
cohort of persons known to have had exposure.
OBTAINING DATA ON EXPOSURE
01/09/2015COMMUNITY MEDICINE SEMINAR
9
To know whether they are exposed or not, and also the
degree of exposure. Various methods used are:
 Directly from cohort members
 Review of records
 Medical examination or special tests
 Environmental surveys
SELECTION OF COMPARISON GROUPS
01/09/2015COMMUNITY MEDICINE SEMINAR
10
 Internal comparisons- several sub groups based on
level of exposure
 External comparisons- like smokers and non
smokers
 Comparison with general population rates-if it is
available
FOLLOW-UP
01/09/2015COMMUNITY MEDICINE SEMINAR
11
Regular follow up of all participants to obtain data for
assessing the outcome. Few methods are-
 Periodic medical examination
 Review records
 Routine surveillance of death records
 Mailed questionnaires , telephone calls, home visits.
ANALYSIS
01/09/2015COMMUNITY MEDICINE SEMINAR
12
 Whether any association exists between the
suspected factor and the disease under study
 If so, what is the strength of association
COHORT DISEASE + DISEASE - TOTAL
EXPOSED a b a+b
NOT
EXPOSED
c d c+d
INTERPRETATION
01/09/2015COMMUNITY MEDICINE SEMINAR
13
REFERENCE
01/09/2015COMMUNITY MEDICINE SEMINAR
14
 Park’s Textbook of PREVENTIVE AND SOCIAL
MEDICINE, 23rd edition
 Textbook of community medicine by
AH.SURYAKANTHA, 3rd edition
Thank you
01/09/2015COMMUNITY MEDICINE SEMINAR
15

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Steps in cohort study

  • 2. CONTENTS 01/09/2015COMMUNITY MEDICINE SEMINAR 2  Introduction  What is a cohort?  Elements of a cohort study  Selection of study subjects  Obtaining data on exposure  Selection of comparison groups  Follow up  Analysis  References
  • 3. INTRODUCTION 01/09/2015COMMUNITY MEDICINE SEMINAR 3  Cohort study is a type of analytical study  Done to support or refute the existence of an association between suspected cause and disease  Prospective / longitudinal /incidence / forward looking study  CAUSE → EFFECT  Types-Prospective, Retrospective, Combination
  • 5. WHAT IS A COHORT ??? 01/09/2015COMMUNITY MEDICINE SEMINAR 5  COHORT-a group of people who share a common characteristic or experience within a defined time period.  STUDY AND CONTROL COHORTS  Both should be free of disease, equally susceptible to the disease, and both groups should be comparable to each other.
  • 6. FRAMEWORK OF COHORT STUDY 01/09/2015COMMUNITY MEDICINE SEMINAR 6
  • 7. ELEMENTS OF A COHORT 01/09/2015COMMUNITY MEDICINE SEMINAR 7 1.Selection of study subjects 2.Obtaining data on exposure 3.Selection of a comparison group 4.Follow up 5.Analysis
  • 8. SELECTION OF STUDY SUBJECTS 01/09/2015COMMUNITY MEDICINE SEMINAR 8 • General population- If exposure is frequent, an appropriate sample size(exposed and unexposed) is taken from general population. • Special groups - - like selected profession, college alumini, antenatal women, etc. -exposure groups-if exposure is rare, select a cohort of persons known to have had exposure.
  • 9. OBTAINING DATA ON EXPOSURE 01/09/2015COMMUNITY MEDICINE SEMINAR 9 To know whether they are exposed or not, and also the degree of exposure. Various methods used are:  Directly from cohort members  Review of records  Medical examination or special tests  Environmental surveys
  • 10. SELECTION OF COMPARISON GROUPS 01/09/2015COMMUNITY MEDICINE SEMINAR 10  Internal comparisons- several sub groups based on level of exposure  External comparisons- like smokers and non smokers  Comparison with general population rates-if it is available
  • 11. FOLLOW-UP 01/09/2015COMMUNITY MEDICINE SEMINAR 11 Regular follow up of all participants to obtain data for assessing the outcome. Few methods are-  Periodic medical examination  Review records  Routine surveillance of death records  Mailed questionnaires , telephone calls, home visits.
  • 12. ANALYSIS 01/09/2015COMMUNITY MEDICINE SEMINAR 12  Whether any association exists between the suspected factor and the disease under study  If so, what is the strength of association COHORT DISEASE + DISEASE - TOTAL EXPOSED a b a+b NOT EXPOSED c d c+d
  • 14. REFERENCE 01/09/2015COMMUNITY MEDICINE SEMINAR 14  Park’s Textbook of PREVENTIVE AND SOCIAL MEDICINE, 23rd edition  Textbook of community medicine by AH.SURYAKANTHA, 3rd edition