Cohort Study Ram Rangsin, MD MPH DrPH Department of Military and - PowerPoint PPT Presentation

Cohort Study Ram Rangsin, MD MPH DrPH Department of Military and Community Medicine Phramongkutklao College of Medicine, THAILAND 1

Epidemiological Studies • Describe the problem – How large is the problem? – How dose the problem distribute in the population – Descriptive Study • Time • Place • Person 2

Epidemiological Studies Epidemiological Studies • Identify factors associated with the problem – Cross-sectional – Case-control – Cohort 3

Two steps of research questions • Size of the problems – Prevalence of HT in the population • Association of the problems – Did the HT in this population relate to the education levels? 4

Classification epidemiological study Classification epidemiological study Experiment Experiment Observation Observation (exposure given by researcher) (exposure given by researcher) (natural exposure) (natural exposure) Descriptive Analytic Descriptive Analytic (no comparison group) (with comparison group) (no comparison group) (with comparison group) Cross- -sectional sectional Case control Cohort Cross Case control Cohort Cross-sectional Case control Cohort 5

Hierarchy of Hierarchy of Epidemiological studies Epidemiological studies • Case report: Descriptive Descriptive Descriptive • Case series: • Cross-sectional study: • Cross-sectional study: • Case-control study: Analytic Analytic Analytic • Cohort study: • Clinical trial: Experiment Experiment Experiment 6

Cohort Study Cohort Study • The most powerful observational study for identifying an association of risk factors and a disease • The most time consuming • The most expensive 7

8 A unit of 300-600 men in the ancient Roman army “COHORT” “COHORT”

A Roman Cohort Two centuries made one maniple and three maniples made up one cohort. 9

“ COHORT ” “ COHORT ” in Epidemiology in Epidemiology A group of persons who are A group of persons who are followed over time followed over time 10

Cohort Study Cohort Study • Start with a group of people with out the disease • Then divide people on the basis of the exposure to a suspected risk factor • Follow the “whole group” for a period of time • Then asses the disease occurrence outcome 11

Using epidemiology to identify Using epidemiology to identify the cause of disease the cause of disease Effect Effect Effect Cause Cause Cause • RISK FACTOR RISK FACTOR DISEASE • • DISEASE • • RISK FACTOR • DISEASE • Cigarette Cigarette Lung Cancer • • Lung Cancer • • Cigarette • Lung Cancer 12

Effect Cause Cause Effect Effect Cause Factors Disease Factors Disease Case- -Control Control Case Factors Disease Factors Disease Factors Disease Cohort Cohort Cohort 13

Factors Factors Factors Disease Disease Disease Exposed Not Exposed Exposed Not Exposed Exposed Not Exposed Develop Develop Develop Do Not Develop Do Not Do Not Do Not Develop Develop Do Not Do Not Disease Disease Disease Develop Disease Develop Develop Develop Disease Disease Develop Develop Disease Disease Disease Disease Disease Disease 14

Type of Cohort Study I. Concurrent Cohort Study (Prospective Cohort Study) Exposed Not Exposed Exposed Not Exposed Exposed Not Exposed 2005 5 200 Develop Develop Develop Do Not Develop Do Not Do Not Do Not 2015 5 Develop Develop 201 Do Not Do Not Disease Disease Disease Develop Disease Develop Develop Develop Disease Disease Develop Develop 15 Disease Disease Disease Disease Disease Disease

Type of Cohort Study Type of Cohort Study II. Retrospective Cohort Study (Take advantage of records collected) Exposed Not Exposed Exposed Not Exposed Exposed Not Exposed 1970 1970 Develop Develop Develop Do Not Develop Do Not Do Not Do Not 2005 5 Develop Develop 200 Do Not Do Not Disease Disease Disease Develop Disease Develop Develop Develop Disease Disease Develop Develop 16 Disease Disease Disease Disease Disease Disease

Persons without the disease!!!! Persons without the disease!!!! Persons without the disease!!!! 1970 1970 1970 Not Exposed Not Exposed Not Exposed Exposed Exposed Exposed Not Smoke Not Smoke Smoke Not Smoke Smoke Smoke # 500 persons # 500 persons # 500 persons # 500 persons # 500 persons # 500 persons Disease No Disease 2005 5 Disease No Disease Disease No Disease 200 Disease No Disease Disease No Disease Disease No Disease 2005 No Lung Cancer Lung Cancer No Lung Cancer Lung Cancer No Lung Cancer Lung Cancer No Lung Cancer Lung Cancer No Lung Cancer Lung Cancer No Lung Cancer Lung Cancer # 455 # 1 # 499 # 45 # 455 # 455 # 1 # 1 # 499 # 499 # 45 # 45 17

CA Lung No CA CA Lung No CA Smoke 45 Smoke 455 45 455 500 500 499 1 499 1 Not smoke Not smoke 500 500 • Incidence of Smoker who develop Lung Cancer = 45 /500 Incidence of Smoker who develop Lung Cancer = 45 /500 • • Incidence of Smoker who develop Lung Cancer = 45 /500 • Incidence of Non Incidence of Non - -Smoker Smoker whodevelop whodevelop Lung Cancer = 1 /500 Lung Cancer = 1 /500 • • Incidence of Non -Smoker whodevelop Lung Cancer = 1 /500 45/500 • Relative Risk of smoking for Lung Cancer = = 45 Relative Risk of smoking for Lung Cancer = = 45 • • Relative Risk of smoking for Lung Cancer = = 45 1/500 • Those who smoked were Those who smoked were 45 times more likely 45 times more likely to get to get • • Those who smoked were 45 times more likely to get lung cancer lung cancer lung cancer 18

Relative Risk Relative Risk CA Lung No CA CA Lung No CA A Smoke A A+ B Smoke B A+ B A+ B B C D C D Not smoke C+ D Not smoke C+ D C+ D Relative Risk = A/ A+ B A/ A+ B Relative Risk = C/ C+ D C/ C+ D 19

Interpretation of Relative Risk (RR) Interpretation of Relative Risk (RR) • Relative Risk of smoking for CA Lung = 45 • Those who smoked were 45 times more likely to develop lung cancer than those who did not smoke. 20

Cohort Study Cohort Study Comparison between “a group of persons with a factor -- Exposed” VS “a group of persons without the factor -- Non-exposed” 21

Measurement of Associations • Cross-sectional Point Prevalence Rate Ratio Point Prevalence Rate Ratio Point Prevalence Rate Ratio • Case-Control Odds Ratio Odds Ratio Odds Ratio Relative Risk Relative Risk Relative Risk • Cohort 22

Advantages Advantages of cohort study of cohort study • Temporal sequence (exposure occur prior the disease) can be more clearly established • Well suited for assessing the effect of RARE EXPOSURE (e.g. Radiation,) – Persons are enrolled on the basis of exposure 23

Advantages Advantages of cohort study of cohort study • Able to examine multiple diseases outcome of a single exposure – The Nurse Health Study, USA – 120,000 female nurses – Exposure: Oral Contraceptive Pill – Outcomes: • Breast cancer • Ovarian Cancer • Malignant melanoma 24

Disadvantages Disadvantages • Insufficient for the evaluation of rare diseases • Extremely expensive and time consuming (Prospective) • Required the availability of adequate records (Retrospective) • Loss to follow-up 25

When we are conducting a cohort study, we are dealing with “INCIDENCE”. 26

Death Death Cured Cured 1994 1994 1996 1996 JAN MAY JUL SEP DEC JAN MAY JUL SEP DEC 1995 1995 1995 1995 1995 1995 1995 1995 1995 1995 27

Death Death Cured Cured JAN MAY JUL SEP DEC 1995 1995 1995 1995 1995 Incidence in 1995 = ? Incidence in 1995 = ? Point Prevalence at July 1995 = ? Point Prevalence at July 1995 = ? 28

29 Deaths Deaths Cures Cures Incidence Prevalence Prevalence Incidence

30 Prevalence Prevalence I ncreased I ncreased Incidence Incidence Prevalence Prevalence Baseline Baseline

31 Prevalence Prevalence Decreased Decreased Deaths Deaths Cures Cures Prevalence Prevalence Baseline Baseline

Rate Rate • The central tool of Epidemiology is the comparison of RATES • RATE = Numerator Denominator – Mortality Rate – Prevalence – Incidence 32

Measuring the rate of incidence Measuring the rate of incidence There are two ways of measuring There are two ways of measuring 1) Cumulative incidence = number of new case in specified time population at risk in specified time = 40 = 1.25 / 1,000 32,000 33

Measuring the rate of incidence Measuring the rate of incidence 2) I ncidence density or I ncidence rate 2) I ncidence density or I ncidence rate More detail calculation can be made when the population under study is lost to follow-up, or dead or turn to disease. This incidence rate is called “ I ncidence density” • Adding Adding “ “TI ME Dimension TI ME Dimension” ” into the denominator into the denominator “Person Person- -time time” ” “ • Person-month, Person-year • 1 Person-year = Following 1 person for 1 year period • 10 Person-year = Following 1 person for 10 year period = Following 10 persons for 1 year period 34