Cohort study Practical lecture

COHORT STUDY

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

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

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
Describe the disease by
Time
Place
Person





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
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,

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

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










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

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

Selection of study subjects
Obtaining data on exposure
Selection of comparison group
Follow up
Analysis

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
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

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
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

Calculation of incidence rates among exposed and non exposed groups

Estimation of risk

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

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
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

• 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


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

Prospective cohort study
Retrospective (historical) cohort study
Combination of Retrospective and Prospective cohort study.

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

THANK YOU