Cohort study - Community Medicine - Lecture Slides, Slides for Medicine. Allahabad University
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Cohort study - Community Medicine - Lecture Slides, Slides for Medicine. Allahabad University

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Medical science is vast field. When we talk about word medicine, we dive into sea of disease details. These lecture slides are not for a medical student only, but these are useful to everyone for general knowledge as wel...
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Cohort study

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Contents

 Definition of cohort

 Design(رسم فني) of study

 Selection of Study Cohort and Control Cohort

 Calculation of Incidence rates

 Calculation of Relative risk

 Calculation of ‘P’ value

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Cohort

 Is a group of people who share a common

characteristic or experience

People born on a same day

Students who joined college in a year

Students from Ajilat, students from Zawia

People doing same work e.g. doctors

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Cohort study

 Also called Prospective(مرتقب) study or

Incidence study

 Is usually done after doing Case-Control

study to get more proof of the cause of

disease

 The study is done on people before the

disease occurs

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Design of Cohort study

Disease + Disease -

Study cohort (Risk factor +)

a b a+b

Control cohort

(Risk factor -) c d c+d

a+c b+d

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Design of Cohort study

a b a+b

c d c+d

a+c b+d

Study Cohort

Risk factor +

Control Cohort

Risk factor +

Disease + Disease -

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Elements of a Cohort study

1. Selection of study Cohort

2. Selection of comparison (Control) Cohort

3. Data collection and Follow-up

4. Analysis and interpretation

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Selection of Study Cohort

 They are selected from general population

or from specific groups e.g. Doctors,

students etc.

 Members of the study cohort must NOT

have the disease

 Members of the study cohort must be

exposed to the risk factor

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Selection of Control Cohort

 They are selected from general population or

from specific groups e.g. Doctors, students etc.

 Members of the control cohort must NOT have

the disease

 Members of the control cohort must NOT be

exposed to the risk factor

 Members of the control cohort must be similar to

the study cohort in age, sex etc.

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Follow up

 Both the Study cohort and Control cohort is followed up to see how many develop the disease

 This is done by

Medical examination

Personal visit, Phone call etc.

 Follow up is difficult because some persons will not respond(أجاب)

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Analysis of Cohort study

 Incidence rate of disease among Study

cohort is calculated

 Incidence rate of disease among Control

cohort is calculated

 Then Relative Risk is calculated

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Smoking and Lung cancer

7000

a+b

3000

c+d

a+c b+d

Study Cohort

Smoking +

Control Cohort

Smoking -

Disease +

Lung cancer +

Disease –

No Lung cancer

70

(a)

6930

(b)

3

(c)

2997

(d)

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7000

a+b

3000

c+d

a+c b+d

Study Cohort

Smoking +

Control Cohort

Smoking -

Disease +

Lung cancer + Disease –

No Lung cancer

70

(a)

6930

(b)

3

(c)

2997

(d)

Incidence rate among smokers = 70/7000 = 10 per 1000

Incidence rate among non-smokers = 3/3000 = 1 per 1000

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Relative risk =

Incidence of disease among exposed

Incidence of disease among non-exposed

10/1= 10

 Relative risk of 10 means that those who smoke

have 10 times more risk of developing lung

cancer than those who are not smoking

 If relative risk is 1, that means there is no risk

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P value

 There are many millions of cigarette

smokers

 We have studied only 7000

 We have to know whether the Relative

Risk of 10 is by chance or not

 We calculate the P value. If the P value is

less than(<) 0.05 we accept the Relative

Risk

 P value in our Lung cancer study is <0.001 Docsity.com

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This result is not by chance(تصادفي) because P-

value is < (less than) 0.05

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Advantage of Cohort study

 Incidence of disease can be calculated

 More than one disease due to the risk

factor can be studied

Smoking and Lung cancer, peptic ulcer,

Coronary heart disease etc.

 Gives better proof of the risk factor than

Case Control study

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Disadvantages

 It takes long time to complete study

 Persons may lose interest and will not come for follow-up

 The person who is doing the study may lose interest or take another job

 Cohort studies are expensive

 Has more ethical problems

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Summary

 Cohort study gives better proof of the cause of disease

 A group of people (Study Cohort) with the risk factor is selected

 Another group of people (Control Cohort) without the risk factor is selected

 Both groups are followed up to see how many develop disease

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Summary

 Incidence rate of disease is calculated

among study cohort

 Incidence rate of disease is calculated

among the control cohort

 Relative risk is calculated

 Cohort study is more difficult and

expensive than Case Control study

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