Cohort study:

PROSPECTIVE STUDY, LOGITUDINAL STUDY, INCIDENCE STUDY & LOOKING FORWORD STUDY.

FEATURES OF COHORT STUDY

The cohort are identified prior to the appearance of the disease under study

The study group so defined are observed over a period of time to determine the frequency of the disease among them.

Cause to effect

Concept :

by definition, a group of people who share a common characteristic or experience within a defined time period.E.g. age, occupation, exposure to a drug or vaccine, pregnancy.The comparison group may be general population from which the cohort is drown, or it may be another cohort of a persons thought to have had little or no exposure to the substance in question, but otherwice similar

Indications:

When there is good evidence of an association between exposure and disease, as derived from clinical observations and supported by descriptive & case control studies.When exposure is rare, but the incidence of the disease is high among the exposure group like those in industries, exposure to x rays ect.When attrition of the population can be minimized, e.g. follow up is easy, cohort is stable, co operative and easily accessible.When ample funds are available.

Frame workEffect to cause…… exposure has occurred but the disease has not.

cohort Disease

Yes No

Total

Exposed to

Etiological factor

a b a+ b

Not exposed to Etiological factor

c d c+ d

Types of cohort study:

Prospective cohort study

Retrospective cohort study

A combination of retrospective and prospective cohort study

Three types of cohort study have been distinguished on the basis of the time of the occurrence of the disease in relation to the time at which the investigation is initiated and continued.

Prospective cohort study/current

The out come not occurred at the time the investigation begins

Most study are begin in present and continue into future.

E.g. smoking and the lung cancer.

Retrospective study/ historical

The out come all occurred before the start of the investigation.

the investigation goes back in time, some times 10 yrs to 30 yrs.

to select these groups from existing records of past employment, medical records, ect.

A combination of retrospective and prospective cohort study

The cohort is identified from the past records, and it is assessed of date for out come. The same record is followed up prospectively in to the future for further assessment.

ELEMENTS OF COHORT STUDY

SELECTION OF STUDY SUBJECTS

OBTAINNING DATA ON EXPOSURE

SELECTION OF COMPARISION GROUPS

FOLLOW UP

ANALYSIS

SELECTION OF STUDY SUBJECTS

GENERAL POPULATION

Special groups:

A) select groups:

B) Exposure group

OBTAINNING DATA ON EXPOSURE

Cohort members: personal interview, or mailed questionnaire

Review of the records

Medical examination or special test

Environmental survey

SELECTION OF COMPARISION GROUPS

Internal comparison

External

Comparison with general population rates

FOLLOW UP

Regular follow up of participant? Depending on the out come to determined. (morbidity or mortality).Periodic medical examination of each member of the cohortReviewing physical & hospital recordsRoutine surveillance of death recordsMailed questionnaire, telephone calls, periodic home visits……

Drawback:

Migration, change of residence or withdrawal from occupation.

These loses may bias the result

ANALYSIS

The data are analyze in terms of:

a) Incidence rates of outcome among and non exposed.

b) Estimation of risk:

i) relative risk

ii) population attributed risk

Advantages

Incidence can be calculatedSeveral possible outcomes related to exposure can be studied simultaneously---- i.e. we can study the association of suspected factor

with many other diseases in addition to one under studyCohort study provide a direct estimation of relative risk dose response ratio can be calculatedSince comparison groups are formed before the disease develops, certain forms of bias can be minimize like misclassification of the individuals in to exposed and unexposed groups.

Disadvantages

Large population, they are generally unsuitable for investigating uncommon disease or disease with low incidence in the population. it takes long time to complete the study and to obtain the result ( 20-30 yrs or more in cancer studies) by which time the investigators may died or participants may have change there classification. it is difficult to keep a large number of individuals under medical surveillance indefinitely.Administrative problems such as loss of funding, loss of experienced staff, extensive record keeping are inevitableLose of interest, migrationSelection of comparison groups which are representative of the exposed & unexposed segments of the population is a limiting factor.ExpensiveAlter people’s behavior

case control cohort study.Effect to causeStart with the disease

Test the suspected cause occurs more frequently in those with the disease than among those without the diseaseUsually the first approach to the testing of a hypothesis, but also useful for exploratory studiesInvolves fewer number of subjectsYields relatively quick resultsSuitable for the study of rare diseases

Relatively inexpensive

Cause to effectStarts with the people exposed to risk factor or suspected causeTest whether the disease occurs more frequently in those exposed, than in those not similarly exposedReserved for testing of precisely formulated hypothesisInvolves large number of subjectsLong follow up period often needed, involving delayed results.Inappropriate when the disease or exposure under investigation is rareexpensive