Transcript ANALYTICAL STUDIES

ANALYTICAL STUDIES
Prospective Studies
COHORT
Prepared by: Dr. Sahar Sabbour
Community Medicine Department
Points discussed
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Types of analytic studies
Aim of analytic studies
Flow chart of the design
Types of cohort studies
Analysis of results
Examples from the literature
Advantages & disadvantages
Intended Learning Outcomes
Students should be able to:
• List types of analytical studies
• Define cohort, identify types of cohort studies
• Describe the prospective approach
• Investigate problems using the prospective
design
• Draw a flow chart showing the cohort study
• Compare between cohort and case control
studies
• Calculate rates from cohort studies
Analytical Studies
Introduction:
Analytical studies are either:
• Observational
Case-Control
Cohort Study
• Experimental (Intervention):
Animal Experiments
Human
Therapeutic trials
Preventive trials
Analytic Studies
• Analytic studies, etiologic studies, are
performed to test specific hypothesis
about a specific health problem.
• In general, associations observed in
descriptive studies are often the basis for
gathering more specific data and testing
hypothesis in additional studies.
• Analytic studies involve the selection and
comparison of two or more groups of
persons, based on either their exposure
or disease status…. WHY?
• To evaluate an association between
exposure and disease.
• Analytic studies focuses on the
magnitude of the association between
the exposure and the health problem
under the study.
A fourfold table
Retrospective
(Cohort)
Prospective
(Case-Control)
E
X
P
O
S
U
R
E
cases
present
present
exposed
absent
Not exposed
controls
DISEASE absent
a
b
c
d
Total
Total
Mausner, 1985
• The difference between the two types of studies
lies in the way the study groups are assembled
• With either method of study, if there is a
positive association between the factor and the
disease:
• Those exposed will tend to develop the disease
(group a),
• Those not exposed will tend not to develop it
(group d).
The Prospective Approach
• The general concept of a prospective
study is relatively simple.
• This type of study has been described by
a variety of items:
-Cohort
-Incidence
-Longitudinal
-Forward looking
-Follow-up
Prospective Cohort
“concurrent”
• COHORT
Study
Retrospective Cohort
“non-concurrent”
Historical prospective
The Prospective Approach
(cont.)
• 1. It starts with a group of people (a cohort) all
considered to be free of a given disease.
• Information is obtained to determine persons
having a particular characteristic (certain
exposure) that is suspected of being related to
the development of disease being investigated.
• 2. These individuals are then followed for a
period of time to observe who develops/or dies
from that disease
• 3. Incidence or death rates for the disease are
then calculated.
The Prospective Approach
(cont.)
• 4. Rates are compared for those with the
characteristic and those without it.
• 5. If the rates (of development of disease) are
different, an association can be said to exist
between the characteristic (exposure) and the
disease.
• 6. It is important to obtain information on other
characteristic of the study groups: age, sex, …
to account for an influence of any factors related
to the disease.
What is a cohort ?
• A cohort is a group of persons who share
a common experience within a defined
time period.
Example:
• Birth cohort, marriage cohort, occupational
cohort
Cohort Study (cont.)
Essential points:
• Exposed individuals in the study should be
representative of all exposed persons.
• Unexposed persons should be
representative of all unexposed persons in
the population.
Cohort Study (cont.)
Selection of Cohorts: several approaches
• Accessible group (volunteers)
• Group with available records/history of
exposure
• Group experiencing some particular
exposure (arising during work)
Cohorts may be heterogeneous or
homogenous
• Heterogeneous: with respect to some
previous exposure as study of lung cancer
and smoking.
• Homogenous in exposure:
As study of the frequency of cancer among
asbestos workers.
The comparison group is the general
population values
Demonstrate excess in deaths among
asbestos workers.
Cohort Study
(Prospective Design)
Passive smoking & respiratory infections
in children
• Is passive exposure to tobacco smoke
associated with increased respiratory
infections in children ?
• Design:
Children exposed and not exposed tobacco
smoke in their homes
Follow them in time for disease occurrence.
Start
Outcome
Family smoker
500 children
Diseased
300
Exposed
Not diseased
Children
(<12 yrs)
1000
1 year
Family non-smoker
500 children
200
Diseased
120
Not exposed
Not diseased
380
Rate: Incidence rate
•Incidence of Resp. Infection among exposed
children:
300
500
= 60%
•Incidence of Resp. Infect. Among non exposed
children:
120
500
= 24%
Cohort
Study
(cont.)
Relative Risk: Incidence rate among exposed
Risk Ratio
Incidence rate in non exposed.
60
24 =
2.5
Relative Risk is a direct measure of risk
(to assess the etiologic role of a factor in
disease occurrence).
300
500
x 500
120
Cohort Study (cont.)
Relative Risk:
Smoking
- Lung Cancer mortality:
RR=18.57
- Myocardial infarction mortality: RR=1.35
It measures the strength of association
Attributable Risk: The absolute
Cohort
Study
(cont.)
difference in Incidence rates among
groups.
“Risk Difference” RD
60 - 24
=
36%
The extent to which the incidence of disease
can be attributed to the risk factor
Smoking
-Lung cancer mortality:
RD=1.23
-Myocardial infarction mortality RD=1.25
Exposure
Category
Annual Death Rates / 100,000 persons
Lung Cancer
Heavy smokers
Nonsmokers
Coronary Heart D.
166
599
7
422
Measures of
Excess Risk
Relative Risk:
166 / 7 = 23.7
599 / 422 = 1.4
Attributable risk:
166 – 7 =159
599 – 422 = 177
Doll and Hill study : Mortality of British doctors cited from Mausner, 1985
• The previous table suggests that
prevention of coronary heart disease
would require alteration of other factors in
addition to smoking.
• The population attributable risk: relates
both relative risk and frequency of the
factor in the population
• i.e. a large proportion of the deaths from
lung cancer in the total population are due
to smoking not only because of the high
RR associated with smoking, but also bec
large proportion of the pop that smoke.
Examples from the literature
• Framingham Heart Study
initiated in 1948 by US Public Health
Services: to study the relationship of a
variety of factors to the subsequent
development of heart disease
Group of persons
30 – 62yrs
6,500
Both sexes
Information:
S. cholest.level
Bl.pressure , weight
Cig. Smoking
20 years follow up
outcome
Occupation Based Studies
to study effect of
exposures
•
•Benzene workers and Leukemia
• Coke-oven workers and lung cancer
•Asbestos workers and lung cancer
•Radium dial painters and oral cancer
Initial Serum Cholesterol
Level
200 - < 220
220 - < 240
320 - < 340
340 - < 970
Males
(45-54y)
RR
1.35
1.48
2.85
3.25
There is an increasing risk of CHD with increasing initial
Serum cholest. Levels in the 45-54 age group from a relative
Risk of 1.13-3.25 M, 1.13-2.89 F
Females
(45-54)
RR
1.3
1.43
2.57
2.89
Advantages of Cohort Study
• Correct classification of exposure before
disease develops.
• Permits calculation of incidence rates thus,
a direct measure of relative risk, and
attributable risk.
• Many possible outcomes to the same
exposure can be studied.
• No chick egg dilema
• Accurate
Disadvantages of Cohort Study
• Large number of people are needed (large
scale).
• Time consuming (follow up)
• Losing people in follow up (Attrition)
• Expensive
• Status of subjects may be changed leading
to error in classification of exposure eg.
Change in habit, occupation.
• Administrative problems: loss of staff,
funding, high costs of the extensive record
keeping
Non concurrent studies
Retrospective Cohort
• The period of observation starts from
some date in the past.
• They usually involve specially exposed
groups or industrial populations.
• Done by using company records of past &
present employees:
• Information: - date of employment
- date of departure - duration, degree of
exposure
- status: living/dead