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

In The Name Of GodTopic: COHORT Studies

Presenter: Armin Hirbod-Mobarakeh

Armin Hirbod-Mobarakeh

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 paradigmArmin Hirbod-Mobarakeh

Definition of cohort study

• Definition of term• Term in research studies


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


Cohort Study Design

Exposed

Not Exposed Disease

No Disease

No Disease

Population

Disease

People Without Disease




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


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


Types Of Cohort Studies

1. Timing• Prospective

• retrospective

2. Open and closed cohorts• Open - people moving in and out

• Closed - fixed population


time

Exposure Study startsDisease

occurrence

Prospective cohort study

time

ExposureStudy startsDisease

occurrence


Retrospective cohort studies

Exposure

time

Diseaseoccurrence Study starts



Measures in Cohort Studies– Cumulative incidence – Incidence density– Risk ratio – Odds ratio


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


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


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


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.


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 • = 10 cases per 100 person-years


Risk ratio

• The relative risk is simply the ratio of the two conditional probabilities.

CasesNon-cases Totals

Exposed A B A+B

Not Exposed C D C+D

Totals A+C B+D

Cohort Analysis:

RR = A/[A+B] C/[C+D]Armin Hirbod-

Mobarakeh

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.


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.

CasesNon-cases Totals

Exposed A B A+B

Not Exposed C D C+D

Totals A+C B+D


1000100

Outcome

700300

no cancer

30Not Exposed (non smoke)

70Exposed (smoke)

cancerExposure

OR = ad/bc = 5.44RR = A/[A+B] C/[C+D]=

4.41

Comparing Odds Ratio and Relative Risk



Biases

• Selection bias• Information bias• Confounding bias


Selection Bias

1. Front-End Bias(can be reduced by screening tests)

2. Non-response bias(can be reduced by analyze)

3. Migration bias(can be reduced by analyze)


Information bias

1. Surveillance bias

2. diagnostic bias

3. Analytic bias• can be reduced by blinding


confounding bias

• Example of Confounding1. Night Light and myopia in Quinn's study

2. Heavy coffee drinking and myocardial infarction

• can be reduced by matching



Subgroups of cohort study

• Historical cohort


Historical Cohort Studies

• Cohort formed in the past with period of follow-up ending also in the past

• Example: Atomic bomb blast survivors


Advantages of Historical Cohort Studies

• Easier to create the cohort • Follow-up has already occurred• Less costly and time consuming


Disadvantages of Historical Cohort Studies

• Incomplete data sets• No control over the quality of the

measurements that are available• Incomplete control of confounding



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 sample1. The population that resides in that district is relatively stable compared with

the other districts of Tehran

2. 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;

3. 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

4. the age distribution of the population of district 13 are representative of the overall population of Tehran and Iran


• Phase 1 :1. population survey

2. 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:1. A list of all households under coverage by three health centers in east of

Tehran was prepared

2. households were specified according to each health center

3. the proportion of households under coverage by each health center to the total number of households in the three health centers was determined

4. lists of all households selected were prepared, and their addresses were determined.

5. Samples were selected using the stratified sampling method.


actions

1. Sending invitations :The crude response rate was approximately 57.5%. there was no significant difference between responders and nonresponders .

2. interview and medical examination.:Participants were first familiarized with the study objectives and were asked to sign a written consent

3. 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.

4. 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.


Oh!!!!!!! Finally it’s over


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