Transcript COHORT STUDIES Dr. A. K. AVASARALA MBBS, M.D. PROFESSOR & HEAD DEPT OF COMMUNITY MEDICINE & EPIDEMIOLOGY PRATHIMA INSTITUTE OF MEDICAL SCIENCES, KARIMNAGAR, A.P. INDIA: +91505417 [email protected].

COHORT STUDIES
Dr. A. K. AVASARALA
MBBS, M.D.
PROFESSOR & HEAD
DEPT OF COMMUNITY
MEDICINE & EPIDEMIOLOGY
PRATHIMA INSTITUTE OF
MEDICAL SCIENCES,
KARIMNAGAR, A.P.
INDIA: +91505417
[email protected]
LEARNING OBJECTIVES
LEARNER SHOULD BE ABLE TO KNOW AT
THE END OF THIS LECTURE
• WHAT IS A COHORT STUDY?
• WHEN TO CONDUCT A COHORT STUDY?
• WHAT TYPES OF COHORT STUDIES ARE
AVAILABLE?
• HOW TO TEST THE HYPOTHESIS BY MEANS
OF A COHORT STUDY?
PERFORMANCE OBJECTIVES
LEARNER SHOULD BE ABLE TO
• CONDUCT THE COHORT STUDY FOR
ANY NEW HYPOTHESIS WHICH HE IS
THINKING CAUSALLY RELATED
• APPLY THIS ANALYTICAL METHOD
WHEN SITUATION DEMANDS
COHORT STUDIES
• ANALYTICAL OBSERVATIONAL
STUDIES
• ALMOST SIMILAR TO THE
EXPERIMENTAL STUDIES
WHAT IS A COHORT?
• COHORT IS A GROUP HAVING A
COMMON CHARACTERISTIC
• e.g. A SMOKER’S COHORT MEANS
ALL ARE SMOKERS IN THAT
GROUP
PURPOSE
• THEY TRY TO PROVE THE
CAUSAL INFLUENCE ON THE
DISEASE OUTCOME IN A
FORWARD DIRECTION FROM
CAUSE TO THE EFFECT AND IN
A COMPARATIVE FASHION
INDICATIONS
1. FOR FURTHER CONFIRMATION OF
CAUSAL HYPOTHESIS ALREADY
SUGGESTED BY CASE CONTROL
DESIGN
2. COHORT METHOD IS DIRECTLY
CARRIED OUT WHEN INDEX OF
CAUSAL SUSPICION OF
HYPOTHESIS IS VERY GREAT
COHORT STUDY DESIGN
• A COHORT, WHICH IS EXPOSED TO
A SUSPECTED FACTOR BUT NOT
YET DEVELOPED THE DISEASE, IS
OBSERVED AND FOLLOWED OVER
TIME
• THEN, THE INCIDENCE OF THE
DISEASE, THE EXPOSURE RISK,
THE DISEASE RISK AND THE
POPULATION RISKS ARE
MEASURED DIRECTLY
COHORT STUDY DESIGN
EXPOSURE
/ SUSPECTED
FACTOR
(SMOKING)
DISEASE
DEVELOPED
(LUNG
CANCER
PRESENT)
DISEASE
TOTAL
NOT
DEVELOPED
(LUNG
CANCER
ABSENT)
PRESENT
a
b
a+b
ABSENT
c
d
c+d
a+c
b +d
a+b+c+d
ANALYSIS
1. IN A SIMILAR MANNER BOTH FOR
THE STUDY COHORT AND
COMPARISON COHORT
2. DUE CONSIDERATION TO
• THE LOSS ON FOLLOW-UP
• THE SECONDARY INFORMATION
OBTAINED FROM OTHER SOURCES
(FAMILY MEMBERS AND
NEIGHBORS) WITH REGARD TO
ATTRITION
COMPARISON COHORT
THE COMPARISON CAN BE DONE
1. INTERNALLY,
2. EXTERNALLY OR
3. WITH GENERAL POPULATION
RATES.
INCIDENCE RATES
• INCIDENCE AMONG THE EXPOSED (NEW
CASES AMONG THE SMOKERS) = (A/ A+B)
• INCIDENCE AMONG THE NON-EXPOSED
(NEW CASES AMONG THE NONSMOKERS) = (C/ C+D )
RISKS
• EXCESS RISK (ER)
=
(A /A+B) – (C / C+D)
• RELATIVE RISK OR RISK RATIO
(RR) = [A / (A+B)] / [C / C +D)] (incidence in the
exposed (smokers) / incidence in the non-exposed (nonsmokers)
• Attributable risk (AR = excess risk/ incidence
among the exposed (AETIOLOGICAL
FRACTION)
• Population attributable risk (PAR) = incidence in
the total population minus incidence among the
non-exposed.
METHODOLOGY
SPECIAL GROUPS
STUDY COHORT
(EXPOSED GROUP)
GENERAL POPULATION
COMPARISON COHORT
(NOT EXPOSED)
INTERNAL OR EXTERNAL
COMPARISON OR WITH THE GENERAL
POPULATION RATES
FOLLOW UP FOR INCIDENCE
OF NEW CASES
INCIDENCE & RISK MEASUREMENT IN BOTH GROUPS
RR, EXCESS RISK, AR AND PAR
PROSPECTIVE OR FORWARD
LOOKING STUDY
FORWARD
SUSPECTED
CAUSE
EFFECT/
DISEASE
FOLLOW UP STUDY
• COHORTS ARE FOLLOWED
TILL NEW CASES ARE
OBSERVED HENCE THE NAME
TYPES
1. CURRENT COHORT OR PRESENT
COHORT (USUAL COHORT)
2. HISTORICAL COHORT (STUDY
COMMENCED AT A POINT OF TIME IN
THE PAST UNTO THE PRESENT)
3. RETROSPECTIVE – PROSPECTIVE
COHORT (COMMENCED IN THE PAST
AND EXTENDED INTO THE FUTURE)
STEPS IN CONDUCTING
COHORT STUDY
1. FORMATION OF STUDY AND
COMPARISON COHORTS
2. COLLECTING BASELINE INFORMATION
3. FOLLOW-UP
4. ANALYSIS FOR INCIDENCE AND RISK
MEASUREMENT IN BOTH THE GROUPS
AND COMPARISON
FORMATION OF STUDY
COHORT
EXCLUDE FROM THE STUDY
• DISEASED PERSONS
• EXPOSED PERSONS
EXPOSURE CONSIDERATIONS
• HOMOGENEOUS COHORT
• HETEROGENEOUS COHORT
SOURCES OF STUDY COHORT
1. GENERAL POPULATION
2. SOME SELECTED GROUPS LIKE
DOCTORS, LAWYERS, etc
3. SPECIAL EXPOSURE GROUPS
LIKE ASBESTOS WORKERS,
MINERS, etc.
INTERNAL COMPARISON
DIFFERENT SUBGROUPS OF
SAME HETEROGENEOUS
COHORT ARE COMPARED
EXTERNAL COMPARISON
• STUDY COHORT COMPARED
WITH A SEPARATE NONEXPOSED COHORT
COMPARISON WITH
THE GENERAL
POPULATION RATES
IDEAL COHORT
WHILE SELECTING THE COHORT,
•
•
•
•
A STABLE COHORT,
A COOPERATIVE COHORT,
A COMMITTED COHORT AND
A WELL-INFORMED COHORT
IS TO BE USUALLY SELECTED.
COHORT SELECTION
TECHNIQUES
• SPECIAL GROUPS,
• EXPOSURE GROUPS,
• HOSPITAL REGISTERS,
• MEDICAL RECORDS, AND
•DEATH CERTIFICATES, ETC.
COHORT SELECTION
TECHNIQUES
• SELECT BOTH COHORTS FROM SAME
POPULATION
• SELECT THE EXPOSED GROUPS
• MEASURE THE DEGREE AND
DURATION OF EXPOSURE
• DO FOLLOW UP FOR INCIDENCE OF
OUTCOMES.
COLLECTION OF BASE LINE
INFORMATION
• COLLECTED IN A SIMILAR FASHION
FROM BOTH THE GROUPS
• SERVES AS A BENCHMARK FOR
FUTURE ANALYSIS
• TIME, DURATION AND EXTENT OF
COLLECTION OF DATA TO BE
CONSIDERED.
DATA COLLECTION
TECHNIQUES
• THE PERIOD OF RECORDING,
QUALITY AND APPLICABILITY OF
RECORDS
• DOSE-RESPONSE RELATIONSHIP
DATA COLLECTION
TECHNIQUES-2
• SOURCES OF GOOD OR HIGH
QUALITY
• QUESTIONS OF EITHER DIRECT
OR INDIRECT, EQUAL TIME AND
SAME QUESTIONS (USUALLY
OPEN-ENDED)
• INTERVIEWER BIAS
ATTRITION REDUCTION
• BY TAKING THE INFORMED CONSENT
• BY ENLISTING THE COMMITMENT TO
CONTINUE AND COOPERATE IN THE STUDY
• BY TRACING THE LOST ONES AND
INCLUDE THEM
• BY CONSIDERING THE INFORMATION OF
THE LOST PERSONS AT THE TIME OF
ANALYSIS
• BY KEEPING THE NON-RESPONSE AT A
LOW LEVEL TO IMPROVE THE VALIDITY
FOLLOW UP
• WELLDEFINED AND STANDARDIZED
END POINTS FOR PROPER FOLLOW UP
• SIMILAR END POINTS FOR BOTH
COHORTS
• DURATION OF FOLLOW UP
FOLLOW-UP TECHNIQUES
1. PERIODICAL MEDICAL EXAMINATIONS AND MAILED
QUESTIONNAIRES
2. DIRECT PERSONAL INTERVIEWS OR EXAMINATIONS
3. VIDEOCONFERENCE, NEIGHBORS, FRIENDS AND
RELATIVES, ETC
4. LOST PERSONS CAN BE TRACED THROUGH THE
LETTERS, FROM THEIR RELATIVES AND FRIENDS
5. MIGRATED COHORT SUBJECTS CAN ALSO BE TRACED
THROUGH TRAVEL AND IMMIGRATION AUTHORITIES
1. DEAD PERSONS - LOCAL OR REGIONAL MORTALITY
REGISTERS OR DEATH CERTIFICATES
PROBLEMS FOR FOLLOW UP
• THE FOLLOW-UP OF LARGE GROUP
(USUAL IN COHORT STUDY)
• RESOURCE CRUNCH
• TIME SCARCITY
• PAUCITY OF TRAINED PERSONNEL
• ATTRITION, LOSS ON FOLLOW UP
EXAMPLES
1.HISTORICAL COHORT STUDY ON
ARTIFICIAL MENOPAUSE AND BREAST
CANCER IN BOSTON AREA (1940)
2.PROSPECTIVE STUDY ON SMOKING AND
MORTALITY IN BRITISH DOCTORS (1951)
3. FRAMINGHAM HEART STUDY (1951)
ARTIFICIAL MENOPAUSE AND
BREAST CANCER IN BOSTON
AREA (1940)
1ST STEP - FORMATION OF COHORTS
• WOMEN AGED 55 YEARS AND
YOUNGER AND TREATED IN TWO
BOSTON HOSPITALS BETWEEN 1920
AND 1940 ARE SELECTED FOR THE
STUDY. DETAILS WERE COLLECTED
FROM THE PATHOLOGICAL RECORDS
AND DEATH CERTIFICATES.
ARTIFICIAL MENOPAUSE AND BREAST
CANCER IN BOSTON AREA (1940)
II STEP - COLLECTING INFORMATION
• FROM THE AVAILABLE SURGICAL &
PATHOLOGICAL RECORDS OF HIGH
QUALITY
• INFORMATION IS ALSO AVAILABLE
ABOUT THE CO-VARIABLES
PROSPECTIVE STUDY ON SMOKING
AND MORTALITY IN BRITISH
DOCTORS (1951)
1ST STEP - FORMATION OF COHORTS
40637 BRITISH DOCTORS WERE
SELECTED FROM BRITISH
MEDICAL REGISTER.
PROSPECTIVE STUDY ON SMOKING
AND MORTALITY IN BRITISH
DOCTORS (1951)
II STEP- COLLECTING INFORMATION
• MAILED QUESTIONNAIRES WERE USED
FOR OBTAINING INFORMATION.
• ADDITIONAL QUESTIONNAIRES WERE SENT
TO KNOW MORE ABOUT
A) SMOKING, AGE WHEN SMOKING
STARTED, NATURE, DURATION, CURRENT
STATUS
B) IF STOPPED, THE DETAILS
FRAMINGHAM HEART STUDY
(1951)
• 1ST STEP - FORMATION OF COHORTS
• TOTALLY, 5209 INDIVIDUALS WERE
REGISTERED FOR THE STUDY.
FRAMINGHAM HEART STUDY (1951)
II STEP - COLLECTING INFORMATION
• BY INITIAL EXAMINATION AND DETAILED
INTERVIEWS, THE DATA WAS GATHERED.
• 82 PERSONS WHO HAD CARDIOVASCULAR
EVENT AT THE INITIAL EXAMINATION WERE
EXCLUDED OUT OF 5209 SUBJECTS.
• THE REMAINING 5127 SUBJECTS WERE
CLASSIFIED ACCORDING TO THE RISK
FACTORS LIKE HIGH SERUM CHOLESTEROL,
SMOKING, HYPERTENSION, BODY MASS
INDEX AND THE PRESENCE OF OTHER
DISEASES.
HURDLES IN COHORT
STUDIES
• FOLLOW UP
• ATTRITION
• ETHICAL PROBLEMS
SUMMARY
THOUGH COHORT STUDY IS
• TIME CONSUMING
• EXPENSIVE
• LESS BIASED
IT IS BETTER ANALYTICAL METHOD
TO PROVE CAUSAL HYPOTHESIS
AND DIRECT RISK MEASURE MENT
•
•
REFERENCES
• Brian Mac Mahan - Epidemiology principles & methods
• Roger Detels, James Mc Even Oxford Text Book of Public Health
• Maxcy-Rosenau-Last, Public
Health & Preventive medicine