Cohort Cohort StudiesStudies

brahimmahmoodzadehbrahimmahmoodzadehhttp://2580414.blogfa.com

0935228684909352286849a831534021@gmail.com

Cohort

Latin: cohors

– Enclosed yard or company of soldiers

Concept:

• A group of individuals that are all similar in some trait and move forward together as a unit

Cohort

A group of persons who

share a common characteristic

such as age ,occupation

Cohort

An epidemiologic design in which the incidence of a disease (or condition) is compared among exposed and unexposed individuals

PAR = Population at Risk

S = Sampling design

T = Elapsed time

PAR SStudy Group

Exposed

Unexposed

Outcome

NoOutcome

T

Outcome

Cohort Study Design

exampleMortality in relation to smoking:

10 years observation of British docs• Cohort: British doctors responding to a survey in

1950– 65% response rate• Exposed: smokers ( and quantified amount)• Unexposed: non-smokers• Outcome:Lung Ca and death• Results: Increased risk with any smoking & a dose

response relationship

exampleLatency of asbestos among insulation

workers in the US and Canada• Exposed: 17,800 males in Asbestos

Insulation Workers union in North America• Unexposed: General population of males

matched by age• Outcome: lung cancer rates through 1975• Results: Positive assoc. between asbestos&

lung CA

ExampleOxford Family Planning Association Study in

the United Kingdom

Aim: a view of the beneficial and harmful effects of different methods of contraception

17,000 women between 1969 and 1974 in 17 of the largest and best clinics run by the Family Planning Association in England and Scotland

Oxford Family Planning Association Study in the United Kingdom

– met specific eligibility criteria – visits on an annual basis – All events of ill health were recorded, – death and diagnosis of cancer – 10 yr follow-up and +

– Aniline-dye occupational cohort, 1954

Example

• A study of the outcome of delivery subsequent to induced abortion

• historical cohort study

historical cohort study• study of the outcome of delivery subsequent

to induced abortion

• Aim: if an induced abortion increases the risk of pre-term birth or low birthweight in pregnancies following the abortion

• induced abortion in one hospital in Sweden

5,292 induced abortions

cont

• A control group :from the Medical Birth Registry.

• The abortion history of women in the control group was checked from their antenatal care records.

• abortions (exposure):1970-1975

• deliveries (outcomes):1970-1978

• data collection :1978- 1981

Both are longitudinal! Direction of study moves

forward with time.

Cohort created at time Cohort created at time of study initiationof study initiation but but exposure in the exposure in the pastpast and outcomesand outcomes both both past and futurepast and future

exampleMilitary looks toward 1985 in ongoing defoliant study

• Question: Are there deleterious effects of exposure to agent orange in servicemen?

• Exposed: 1264 exposed to defoliant spraying in Vietnam• Unexposed: 1264 who flew cargo missions at same time• Outcomes(retro): Medical problems during exposure time– e.g. dermatologic conditions, birth defects, liver problems• Outcomes (prospective): cancer rates up to 25 years later

Cohort Study DesignTypes of Cohorts

• Fixed Cohort – A group of individuals recruited and enrolled at a

uniform point in the natural history of a disease or by some defining event

– Cohort does not take on new members after it is assembled

– Examples• Patients admitted to the ER with acute MI

• Survivors of Hiroshima bombings

• Children born to HIV-infected mothers

Cohort Study DesignType of Cohorts

• Open cohort– A group of individuals recruited and enrolled

through a mechanism that allows for in and out migration of people

– Defined by characteristic other than disease, e.g., geographic location, administrative unit

– Dynamic population

Deaths

Loss to follow-upFixed Cohort

Time

Open Cohort

In - Migrations

MigrationsOut -

Cohort Study DesignDirectionality

1990 2000 2010

ProspectiveRetrospective

Retrospective Cohort Study

Prospective Cohort Study

Longitudinal

Study Population

• Define Population at Risk using inclusion criteria

• Individuals with outcome of interest at time of screening and enrollment are not eligible for study

• Sub-clinical presentation of diseases may be present challenges in defining the cohort

Study PopulationsExamples

• Framingham study of cardiovascular disease– Individuals 30 – 62 years old in community at

risk for disease– 1948 to present

Study Populations• Multi-Centered AIDS Cohort Study

– Goal to elucidate the natural history of HIV/AIDS

– 5000 gay men, volunteers– 5 cities in US– 1984 – 1999

Measuring Exposure• Exposure measurement must be

comparable for all members of the cohort

• Carefully defined in advance of study

• Specific attention should be given to the accuracy

• Pilot studies often needed

Measuring ExposureMeasurements

– Chart review– Interview– Blood tests or other specimens

• Biomarkers

– Other laboratory tests– Sample storage

Outcome Definition• Primary outcome - the main event that will

be related to the exposure

• Death

• Disease occurrence

• Secondary outcomes - other events that are of interest and may corroborate the findings of the main outcome

Follow-up• Completeness and non-participation

– 90% rule of thumb

• All subjects must have an equal likelihood for detecting the outcome

• Disease ascertainment must be comparable between the exposed and unexposed subjects– Number of visits– Reasons for additional evaluations

Follow-up

• Follow-up mechanisms– Active– Passive

• Passive Surveillance– Hospitals– Disease Registries– Clinics or physician offices– Surveillance systems, e.g., National Death

Index, CDC reportable conditions

Follow-up

• Active surveillance– Systematic evaluations for outcome of interest– Regular time intervals – In all study subjects

Regardless of active or passive surveillance, the persons evaluating subjects must be blinded to exposure status

Cohort Study DesignPotential Biases

• Detection bias – Subjects with exposure are more (or less)

closely evaluated for outcome

• Information bias– The quality of information is different

between exposed and unexposed subjects

• Non-response and loss to follow-up– Selective loss of exposed (or unexposed)

persons

Importance of the comparison group

• cohort studies do not have randomizationdo not have randomization of study subjects

• vulnerable to selection biasselection bias

• Increasing the sizeIncreasing the size of the study can only partially help address the issue

Strengths• Useful for rare exposures• Multiple effects of single

exposure• Temporal relationship between

exposure and outcome• Ascertainment bias minimized• Direct measurement of

incidence

Weaknesses• Inefficient for rare diseases•Expensive•Requires excellent follow-up•Losses to follow-up can

invalidate the study

و • سالم گروه دو در حاملگي عوارض مقايسه : كوهورت مطالعه حاملگي ديابت به مبتال

و • سینه خس خس با همراه بیماریهای رابطهکوچک کودکان در سرفه سیاه واکسیناسیون

نگر لعهمطا: آینده کوهورت

4251

0011 0010 1010 1101 0001 0100 1011

تعيين عوامل خطر سرطان مری در استان •گلستان

سال 10 نفر به مدت بيش از 65000•پيگيری خواهند شد ) ملک زاده و

(1382همکاران،مطالعه • شده 14کل گرفته نظر در سال

است.