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

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Cohort Study

Ram Rangsin, MD MPH DrPH

Department of Military and Community Medicine Phramongkutklao College of Medicine, THAILAND

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Epidemiological Studies

• Describe the problem

– How large is the problem? – How dose the problem distribute in the population – Descriptive Study

• Time
• Place
• Person

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Epidemiological Studies Epidemiological Studies

• Identify factors associated with the

problem

– Cross-sectional – Case-control – Cohort

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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?

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Classification epidemiological study Classification epidemiological study

Observation Observation

(natural exposure) (natural exposure)

Experiment Experiment

(exposure given by researcher) (exposure given by researcher)

Descriptive Descriptive

(no comparison group) (no comparison group)

Analytic Analytic

(with comparison group) (with comparison group) Cross-sectional Cross Cross-

• sectional

sectional Case control Case control Case control Cohort Cohort Cohort

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Hierarchy of Epidemiological studies Hierarchy of Epidemiological studies

Descriptive Descriptive Descriptive Analytic Analytic Analytic Experiment Experiment Experiment

• Case report:
• Case series:
• Cross-sectional study:
• Cross-sectional study:
• Case-control study:
• Cohort study:
• Clinical trial:

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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

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A unit of 300-600 men in the ancient Roman army

“COHORT” “COHORT”

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A Roman Cohort

Two centuries made one maniple and three maniples made up one cohort.

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A group of persons who are followed over time A group of persons who are followed over time

“COHORT” in Epidemiology “COHORT” in Epidemiology

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• 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
• utcome

Cohort Study Cohort Study

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Effect Effect Effect

Cause Cause Cause Using epidemiology to identify the cause of disease Using epidemiology to identify the cause of disease

• RISK FACTOR
• Cigarette
• RISK FACTOR

RISK FACTOR

• Cigarette

Cigarette

• DISEASE
• Lung Cancer
• DISEASE

DISEASE

• Lung Cancer

Lung Cancer

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Effect Effect Effect

Cause Cause Cause

Factors Factors Disease Disease

Case Case-

• Control

Control

Factors Factors Factors Disease Disease Disease

Cohort Cohort Cohort

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Factors Factors Factors Disease Disease Disease

Develop Disease Develop Develop Disease Disease Do Not Develop Disease Do Not Do Not Develop Develop Disease Disease Develop Disease Develop Develop Disease Disease Do Not Develop Disease Do Not Do Not Develop Develop Disease Disease

Exposed Exposed Exposed Not Exposed Not Exposed Not Exposed

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Type of Cohort Study

• I. Concurrent Cohort Study

(Prospective Cohort Study)

Develop Disease Develop Develop Disease Disease Do Not Develop Disease Do Not Do Not Develop Develop Disease Disease

Exposed Exposed Exposed Not Exposed Not Exposed Not Exposed

Develop Disease Develop Develop Disease Disease Do Not Develop Disease Do Not Do Not Develop Develop Disease Disease

200 2005 5 201 2015 5

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Type of Cohort Study Type of Cohort Study

• II. Retrospective Cohort Study

(Take advantage of records collected)

Develop Disease Develop Develop Disease Disease Do Not Develop Disease Do Not Do Not Develop Develop Disease Disease

Exposed Exposed Exposed Not Exposed Not Exposed Not Exposed

Develop Disease Develop Develop Disease Disease Do Not Develop Disease Do Not Do Not Develop Develop Disease Disease

1970 1970 200 2005 5

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Exposed Exposed Exposed Not Exposed Not Exposed Not Exposed

Disease Disease Disease No Disease No Disease No Disease

Smoke # 500 persons Smoke Smoke # 500 persons # 500 persons

Lung Cancer

# 45

Lung Cancer Lung Cancer

# 45 # 45

No Lung Cancer

# 455

No Lung Cancer No Lung Cancer

# 455 # 455

Not Smoke # 500 persons Not Smoke Not Smoke # 500 persons # 500 persons

Disease Disease Disease No Disease No Disease No Disease

Lung Cancer

# 1

Lung Cancer Lung Cancer

# 1 # 1

No Lung Cancer

# 499

No Lung Cancer No Lung Cancer

# 499 # 499

1970 1970 1970 2005 200 2005 5 Persons without the disease!!!! Persons without the disease!!!! Persons without the disease!!!!

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CA Lung CA Lung No CA No CA

Smoke Smoke Not smoke Not smoke

45 45 455 455 1 1 499 499

• Incidence of Smoker who develop Lung Cancer = 45 /500
• Incidence of Non -Smoker whodevelop Lung Cancer = 1 /500
• Relative Risk of smoking for Lung Cancer = = 45
• Those who smoked were 45 times more likely to get

lung cancer

• 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

• Relative Risk of smoking for Lung Cancer = = 45

Relative Risk of smoking for Lung Cancer = = 45

• Those who smoked were

Those who smoked were 45 times more likely 45 times more likely to get to get lung cancer lung cancer 45/500 1/500

500 500 500 500

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Relative Risk Relative Risk

CA Lung CA Lung No CA No CA

Smoke Smoke Not smoke Not smoke

A A B B C C D D

A+ B A+ B A+ B C+ D C+ D C+ D

Relative Risk = Relative Risk = A/ A+ B A/ A+ B C/ C+ D C/ C+ D

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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.

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Cohort Study Cohort Study

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

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Measurement of Associations

• Cross-sectional
• Case-Control
• Cohort

Point Prevalence Rate Ratio Point Prevalence Rate Ratio Point Prevalence Rate Ratio

Odds Ratio Odds Ratio Odds Ratio

Relative Risk Relative Risk Relative Risk

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Advantages

• f cohort study

Advantages

• f cohort study
• Temporal sequence (exposure occur

prior the disease) can be more clearly established

• Well suited for assessing the effect
• f RARE EXPOSURE (e.g. Radiation,)

– Persons are enrolled on the basis of exposure

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Advantages

• f cohort study

Advantages

• f cohort study
• Able to examine multiple diseases outcome
• f a single exposure

– The Nurse Health Study, USA – 120,000 female nurses – Exposure: Oral Contraceptive Pill – Outcomes:

• Breast cancer
• Ovarian Cancer
• Malignant melanoma

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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

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When we are conducting a cohort study,

we are dealing with “INCIDENCE”.

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JAN JAN 1995 1995 DEC DEC 1995 1995 MAY MAY 1995 1995 JUL JUL 1995 1995 SEP SEP 1995 1995 1994 1994 1996 1996

Death Death Cured Cured

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Incidence in 1995 = ? Incidence in 1995 = ? Point Prevalence at July 1995 = ? Point Prevalence at July 1995 = ?

JAN 1995 DEC 1995 MAY 1995 JUL 1995 SEP 1995

Death Death Cured Cured

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Prevalence Prevalence

Deaths Deaths Cures Cures

Incidence Incidence

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Baseline Baseline Prevalence Prevalence

Incidence Incidence

I ncreased I ncreased Prevalence Prevalence

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Deaths Deaths Cures Cures

Baseline Baseline Prevalence Prevalence

Decreased Decreased Prevalence Prevalence

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Rate Rate

• The central tool of Epidemiology is the

comparison of RATES

• RATE = Numerator

Denominator

– Mortality Rate – Prevalence – Incidence

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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

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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

Measuring the rate of incidence Measuring the rate of incidence

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Measuring the rate of incidence

• 2) Incidence density or Incidence rate

= Number of new case in specified time Person-years of observation which is disease free = 40 = 5.0 / 1,000 person-years 8,000 person-years

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Relationship between cigarette smoking and Relationship between cigarette smoking and incidence rate of stroke incidence rate of stroke in a study of in a study of 118,539 population 118,539 population in over 8 years period in over 8 years period

Smoking Smoking

• No. of stroke Person
• No. of stroke Person-
• years

years I ncidence rate I ncidence rate

• f observation
• f observation

/ 100,000 person / 100,000 person-

• years

years

Never Never 70 70 395,594 395,594 17.7 17.7 Ex Ex-

• smoker

smoker 65 65 232,712 232,712 27.9 27.9 Smoker Smoker 139 139 280,141 280,141 49.6 49.6 Total Total 274 274 908,477 908,477 30.2 30.2

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Database of 118,539 subjects

I D Age smoking Stroke Enter Last Contact

Person-Year

1 18 No No 1990 1998 8 2 36 No No 1990 1992 2 3 50 Yes Yes 1991 1998 7 . . . . . . . 118,539 24 Yes No 1993 1998 5

Total 908,477

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Relationship between cigarette smoking and Relationship between cigarette smoking and incidence rate of stroke incidence rate of stroke in a study of in a study of 118,539 population 118,539 population in over 8 years period in over 8 years period

Smoking Smoking

• No. of stroke Person
• No. of stroke Person-
• years

years I ncidence rate I ncidence rate

• f observation
• f observation

/ 100,000 person / 100,000 person-

• years

years

Never Never 70 70 395,594 395,594 17.7 17.7 Ex Ex-

• smoker

smoker 65 65 232,712 232,712 27.9 27.9 Smoker Smoker 139 139 280,141 280,141 49.6 49.6 Total Total 274 274 908,477 908,477 30.2 30.2

Cumulative incidence Cumulative incidence =

=

274/118,539 = 2.31 /1,000 274/118,539 274/118,539 = = 2.31 2.31 /1,000 /1,000

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Exposure assessment

• Exposed VS Non-exposed
• Fixed Exposure
• Time-dependent Exposure(Exposure level

changes through time)

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Fixed Exposure

• Exposure do not change over time

– Sex (Male / Female) – Blood group (A / B / O / AB) – Race (White / Black / Asian) – Expose to radiation from the power plant explosion – Adult height

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Time-dependent

• Exposure level changes over time

– Body weight – Alcohol consumption – Blood pressure level

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Outcome assessment

• Disease
• Specify clearly what is your final outcome

– Disease – Death – Intermediate outcomes

• CD4+ count
• Increased Creatinine

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Conducting a Cohort Study

• Selecting a group of people without the

disease

• Defining the Exposed group
• Defining the Non-exposed group
• Evaluate the disease outcome among

both Exposed and Non-exposed

• Calculating Relative Risk

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• You are interested in the association

between blood cholesterol level and coronary artery heart disease

• Please conduct a cohort study to verify

the association

Conducting a Cohort Study

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• What population would you like to start

with?

Conducting a Cohort Study Conducting a Cohort Study

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• How can you identify those who will be

the “study population”?

Conducting a Cohort Study Conducting a Cohort Study

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• How can you identify exposed and non-

exposed groups?

Conducting a Cohort Study Conducting a Cohort Study

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• What is you follow-up plan?

– What is you outcome of interest? – How often you would like to asses the

• utcome?

– How long will you follow the population?

Conducting a Cohort Study Conducting a Cohort Study

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• What is you plan for the analysis?

– What will be the measurement of association from your study? – What would you like to compare?

Conducting a Cohort Study Conducting a Cohort Study