1. In The Name Of God Topic: COHORT Studies
Armin Hirbod-Mobarakeh
In The Name Of God Topic: COHORT Studies
Presenter: Armin Hirbod-Mobarakeh

2. Targets Definition of cohort study Methodology
Measures in cohort studies
Advantages in cohort study
Disadvantages in cohort study
Types of cohort studies
Biases in cohort studies
Subgroups of cohort study
A paradigm
Armin Hirbod-Mobarakeh

3. Definition of cohort study
Definition of term
Term in research studies
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4. Methodology
Start with persons having the presumed cause (exposure). BUT free from the outcome (disease), and then wait for them to develop the effect
Comparison group - also free from disease, but who, also DO NOT have the presumed cause
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5. Cohort Study Design Exposed Not Exposed Disease People Population
Without
Disease
No Disease
Not Exposed
Disease
No Disease
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8. Advantages of Cohort Studies
Can assess several outcomes
Prospective control over exposure and outcome measurement possible (in prospective studies)
Somewhat less potential for bias than case-control studies, but equal potential for confounding
We can learn about prognosis and etiology of the disease
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9. Disadvantages of Cohort Studies
Not useful for rare outcomes
As an observational study can never be assumed to be free of confounding and bias
Expensive
Time needed to conduct these studies
Why we can not do one follow up
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10. Types Of Cohort Studies
Timing
• Prospective
• retrospective
Open and closed cohorts
• Open - people moving in and out
• Closed - fixed population
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11. Prospective cohort study
Disease
occurrence
Exposure
Study starts
time
time
Exposure
Study starts
Disease
occurrence
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12. Retrospective cohort studies
Disease
occurrence
Study starts
Exposure
time
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14. Measures in Cohort Studies
Cumulative incidence
Incidence density
Risk ratio
Odds ratio
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15. Cumulative Incidence
Cumulative incidence (CI) =is the proportion of people in a population who became diseased or ill or experienced an event during the specified period of time.
CI = No. new cases of disease or events during time period Total population at risk at the beginning of the time period
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16. Cumulative Incidence Example
818 women had in vitro fertilization in NSW during 2000
80 developed a clinical pregnancy within one month of follow-up after the first embryo transfer procedure. The CI of pregnancy was??
CI= 80/818
9.8 cases per hundred women on the program
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17. Incidence Density
The incidence rate or incidence density is the number of new cases in a population divided by the total time units each individual in the population at risk was observed.
Incidence Rate=
No new cases of disease/events during the specified time period
Sum of the length of time during which each person in the population is at risk
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18. Incidence Rate - Example (Incidence Density)
In investigating the incidence of duodenal ulcer following the use of a specific drug in 14 subjects.
4 subjects started the study in Jan 1990 and all finished the study in Dec 1999.
Ten subjects joined the study in Dec1995 and finished the study in November 1996.
During the period of observation: 5 people developed duodenal ulcer while taking the drug.
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19. Incidence Density
What is total length of time that persons were in the population is at risk (denominator)?
4 X 10 years = 40 years
10 x 1 year = 10 years
Total = 50 person-years
New cases = 5
What is the incidence rate of duodenal ulcers after taking the drug?
Incidence rate = 5 / 50
= cases per 100 person-years
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20. Risk ratio
The relative risk is simply the ratio of the two conditional probabilities.
Cases
Non-cases
Totals
Exposed A B
A+B
Not Exposed C D
C+D
A+C
B+D
Cohort Analysis:
RR = A/[A+B]
C/[C+D]
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21. ODDS RATIO
It is defined as the ratio of the odds of an event occurring in one group to the odds of it occurring in another group, or to a sample-based estimate of that ratio.
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22. You can understand the odds ratio by first noticing what the odds are in each row of the table. The odds for row Y- are a/b. The odds for row Y+ are c/d. The odds ratio (OR) is simply the ratio of the two odds.
Cases
Non-cases
Totals
Exposed A B
A+B
Not Exposed C D
C+D
A+C
B+D
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23. Comparing Odds Ratio and Relative Risk
Outcome
cancer
no cancer
Exposure
Exposed (smoke) 70
300
Not Exposed (non smoke) 30
700
100
1000
RR = A/[A+B]
C/[C+D]= 4.41
OR = ad/bc = 5.44
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25. Biases Selection bias Information bias Confounding bias
This classification is by Miettinen OS in 1970s
See for example Miettinen & Cook, 1981
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26. Selection Bias Front-End Bias(can be reduced by screening tests)
Non-response bias(can be reduced by analyze)
Migration bias(can be reduced by analyze)
Consider a study of the effects of exercise on all-cause mortality in persons who are thought to be completely healthy at baseline. Now consider what would happen if some participants with undiagnosed cardiovascular disease (CAD) were indeed enrolled in the study and indeed it turns out that they exercise less (say because they get out of breath more easily).
Selection bias occurs when those persons lost to follow up have a different probability of the outcome than those persons who remain in the analysis
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27. Information bias Surveillance bias diagnostic bias Analytic bias
can be reduced by blinding
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28. confounding bias Example of Confounding can be reduced by matching
Night Light and myopia in Quinn's study
Heavy coffee drinking and myocardial infarction
can be reduced by matching
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30. Subgroups of cohort study
Historical cohort
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31. Historical Cohort Studies
Cohort formed in the past with period of follow-up ending also in the past
Example: Atomic bomb blast survivors
While the Historical Cohort Study is the most well-known type of cohort study, there are other types of cohort studies.
The Incidence Cohort Study assesses the incidence of disease and seeks to identify risk factors for disease onset. It answers the question, “Is the incidence greater in those exposed than in those who are non-exposed?”
The Prognostic Cohort Study follows a diseased cohort to assess factors associated with the outcome. The outcome may be either recovery or death. The primary purpose is to identify all the explanatory factors for the outcome. Such explanatory factors are also known as prognostic factors.
The Randomized Controlled Trial (RCT) can also be considered a special type of cohort studies.
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32. Advantages of Historical Cohort Studies
Easier to create the cohort
Follow-up has already occurred
Less costly and time consuming
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33. Disadvantages of Historical Cohort Studies
Incomplete data sets
No control over the quality of the measurements that are available
Incomplete control of confounding
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35. Paradigm Fereidoon azizi TLGS(Tehran Lipid and Glucose Study)
Sample:The study cohort consists of more than 15,000 individuals who are older than 3 yr at study entry The sampling frame is chosen from urban district 13 of Tehran
The reasons for choosing district 13 as the sample
The population that resides in that district is relatively stable compared with the other districts of Tehran
the medical and health facilities in this district are under the supervision of Shahid Beheshti University of Medical Sciences, which also hosts the Endocrine Research Center, in which the study is designed and managed;
the health centers of district 13 enjoy a well-developed network of experienced volunteers, who play a critical role in the recruitment of individuals for the study
the age distribution of the population of district 13 are representative of the overall population of Tehran and Iran
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36. samples were selected as follows:
Phase 1 :
population survey
baseline health examinations
The primary objectives: measure the population prevalence of known cardiovascular risk factors, including dyslipoproteinemia, hyperglycemia, obesity, smoking, and hypertension.
samples were selected as follows:
A list of all households under coverage by three health centers in east of Tehran was prepared
households were specified according to each health center
the proportion of households under coverage by each health center to the total number of households in the three health centers was determined
lists of all households selected were prepared, and their addresses were determined.
Samples were selected using the stratified sampling method.
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37. actions
Sending invitations :The crude response rate was approximately 57.5%. there was no significant difference between responders and nonresponders .
interview and medical examination.:Participants were first familiarized with the study objectives and were asked to sign a written consent
baseline medical examinations: In this section, trained physicians collected data on the medical history, BP, peripheral pulse, and thyroid examination. Trained technicians obtained anthropometric data such as height, weight, and hip and waist sizes. ECG was taken from those aged 30 yr. Blood samples were drawn after 10 to 12 h of fasting. Laboratory measurements included lipid profiles, fasting blood sugar, thyroid profile, and serum creatinine.
After baseline medical examinations, study participants are being contacted annually by telephone about Medical events. After identification of events, confirmatory data are being collected by study physicians by abstracting hospital records and performing in-home physical examinations.
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38. Oh!!!!!!! Finally it’s over
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