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analytical epidemiology
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Presented by: reMAN dhaKALCODSH-NMCFIRST BATCH
Introduction
Types of epidemiology
Types of analytical epidemiology
Case control study
Cohort study
Comparison between case control and cohort study
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John M. last: "the study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems.
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EPIDEMIOLOGY
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Analytical epidemiology
Second major type of epidemiology. Focus on individual within population unlike descriptive epidemiology.. Objective not to formulate hypothesis but to test hypothesis.
TYPESA. CASE CONTROL STUDYB. COHORT STUDY
Retrospective or trohoc study
Distinct features:
1. Both exposure and outcome have occurred before the start of disease
2. Study proceed backward from effect to cause
3. Uses a control or comparison group to support or refute an inference.
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The basic study design
Control(those without condition)
eg: those free of oral cancer
Cases(those with condition)
eg: cases with oral cancer
Unexposed (without characteristic or risk factor)
Eg. Non chewers
Exposed (with characteristic or risk factor)
Eg. tobacoo chewers
1. Selection of cases and controls
2. Matching
3. Measurement of exposure , and
4. Analysis and interpretation.
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A. Selection of case
Definition of a case:
I). diagnostic criteria:
ii). Eligibility criteria
Sources of cases:
i). Hospital
ii). General population
B. Selection of control
Sources of controls:
i). Hospital controls(common
source of selection bias)
ii). Relatives
iii). General population
Number of
controls/control
groups
Define as:”process by which we select controls in such a way that they are similar to cases with regard to certain pertinent selected variables(eg. Age) which are known to influence the outcome of disease and which, if not adequately matched for comparability, could distort or confounded the result”.
CONFOUNDING FACTOR
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EXPOSURE(eg. Consumption
of alcohol)
DISEASE(eg. Oesophageal
cancer)
CONFOUNDING FACTOR (eg. smoking, age)
Definition and criteria about exposure are just as important as those used to define cases and controls. This may be obtained by :
Interviews
Questionnaries
Studying past record of cases such as hospitalrecords, employment records etc.
Clinical or laboratory examination.
Investigator should not know whether a subject is incase or control group.
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The final step is analysis, to find out:
a) Exposure rates among cases and controls to suspected factors
b) Estimation of disease risk associated with exposure (ODD RATIO)
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Tobacco
chewers
Non-chewers
Cases
(with oral cancer)
33 (a)
2 (c)
Controls
(without oral cancer)
55(b)
27(d)
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Total 35 (a +c) 82 (b +d)
Example of case control study of tobacco chewing and oral cancer
EXPOSURE RATESa. Cases =a/(a+c) = 33/35 = 94.2%b. Controls =b/(b+d) = 55/82 = 67.0%
1. Relative risk = Incidence among exposed
incidence among non exposed
= a c
(a+b) (c+d)
2. Odds ratio (OR) = (a/b)
(c/d)
It measure strength of the association between risk factor and outcome. ,
1. Selection bias :
special types:
a) Prevalence incidence (selective survival)
b) Admission rate ( Berkson/Berkesonian)
2. Information bias:
a) Memory or recall bias
b) Telescopic bias
c) Interviewer’s bias
3. Bias due to confounding
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ADVANTAGES:
1. Relatively easy to carry out.2. Rapid and inexpensive3. Require fewer subjects.4. Suitable for investigation of
rare diseases.5. No risk of subject.6. Allows the study of several
different etiological factors.7. Risk factor can be identify8. No attrition problem
because do not require follow up.
9. Minimal ethical problem.
DISADVANTAGES:
1. Problem of bias since it relies on past memory or past records.
2. Difficulty in selection of appropriate control group.
3. Can not measure incidence only RR.
4. Doesn’t distinguish between cause and associated factors.
5. Not suited for the evaluation of therapy or prophylaxis of disease.
CASE CONTROL STUDY
Prospective ,longitudinal, incidence and forward-looking study
Distinguishing features:
a) The cohorts are identified prior to theappearance of the disease
b) The study groups, so defined, are observedover a period of time to determine thefrequency of disease among them
c) Study proceeds from cause to effect.
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time
Study begins here
Studypopulation
free ofdisease
Factorpresent
Factorabsent
disease
no disease
disease
no disease
present
future
When there is good association between exposure and disease.
When exposure is rare, but the incidence of disease is high among exposed.
When attrition of study population can be minimized.
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1. PROSPECTIVE COHORT STUDY
1. RETROSPECTIVE COHORT STUDY
2. A COMBINATION OF PROSPECTIVE AND RETROSPECTIVE COHORT STUDY.
- Outcome has not yet occurred the time
of investigation begins.
Measure exposureand confounder
variables
Exposed
Non-exposed
Outcome
Outcome
Baseline
time
Study begins here
Outcomes have all occurred before the start of the investigation.
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Measure exposureand confounder
variables
Exposed
Non-exposed
Outcome
Outcome
Baseline
time
Study begins here
1. Selecting of study subject
2. Obtaining data on exposure
3. Selection of comparison group
4. Follow up
5. analysis
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1. Selecting of study subject
When exposure or cause of death is fairly frequent in
the population
i. Select group –Professional
group ( doctors,nurses )
ii. Exposure group-High risk situation
(eg.radiologist exposed to x-ray)
Obtaining data on exposure
Information about exposure may be obtained directly
from:-
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Selection of comparison group
a. Internal comparisons:-
Comparison groups are in built (eg. Smoking, bp etc.) within
same cohort group.
b. External comparisons:- Eg. Smoker and non smoker,
radiologists with opthalmologists.
c. With General population:- If none is available, mortality of
exposed group with general population
Follow up
Main problem Procedures to obtain data
for assessing the outcomeare:
a. Periodic medical checkupb. Reviewing hospital recordsc. Routine surveillance of
death recordsd. Mailed questionnaries,
telephone calls, periodic home visits.
Data are analysed in term of:
a. Incidence rates of outcome among exposed and non-exposed:
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RISK FACTOR (TOBACCO)
DEVELOPEDORAL CANCER
DID NOT DEVELOP
TOTAL
PRESENT (CHEWERS)
45 (a) 9955 (c) 10000 (a + c)
ABSENT(NON CHEWERS)
5 (b) 9995 (d) 10000 (b + d)
5. ANALYSISData are analysed in term of:a. Incidence rates of outcome among exposed
and non-exposed
Incidence rates:1. Among tobacco chewers: = 45/10000 =4.5 per 10002.Among non chewers = 5/10000 = 0.5 per 1000
b. Estimation of risk
A. Relative risk (RR):
= incidence of disease among exposed
incidence of disease among non-exposed
= 4.5/0.5
= 9
It implies 9 times higher risk of development of oral carcinoma in tobacco chewers compared to non chewers.
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B. Attributable risk (AR) Or “risk difference”:
Incidence of disease rate among exposed- incidence among non exposed
Incidence rate among exposed
= 4.5 – 0.5
4.5
= 88.9%
Indicates to what extent the disease under study can be attributed to the exposure.
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1. Selection bias:
Non consent bias
Missing data bias
2. information bias:
Error in classification of individual
Diagnostic bias
3. Confounding bias :
Due to confounding factors
4. Post hoc bias: 30
1. Incidence can be calculated
2. Several possible outcomes related to exposure can be studied simultaneously.
3. Provide a direct estimate of RR.
4. Dose response ratios can be calculated.
5. Since comparison groups are formed before disease develops, certain forms of bias can be minimized like misclassification of individual.
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1. Unsuitable for investigating uncommon disease.
2. Long time to complete study and obtain results.
3. Administrative problem –
Extensive record keeping
4.Expensive
5. Alter people behavior
Stop or decrease smoking
Loss of interest
migration
5. Ethical problem of varying important
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Start disease :effect cause
First approach to testhypothesis.
Involve fewer subject.
Yield result quickly.
Suitable for studying rare disease.
Gives RR only.
Relatively inexpensive. Does not give information about diseases other than that selected for the disease. 33
Start people: cause effect.
Reserved for testing precisely formulated hypothesis.
Involve larger number of subjects. Results are delayed due to long follow up. Unsuitable for studying for rare diseases.
Yield RR and AR.
Expensive Can give information more than one disease.
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