Types of epidemiological studies: Cohort study

A cohort study is a type of observational investigation in which subjects are classified on the basis of the level of exposure to a risk factor and followed to determine subsequent disease outcomes.Cohort study characteristics

  1. Observational study: No manipulation of the study factors
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  2. Have a control group.

  3. From ‘cause’ to ‘outcome’.

l  “Prospective” = forward-looking in time

l  Can be a historical perspective

l  Also called follow-up, incidence, panel, or longitudinal study

  4. Could examine the causal association between the exposure and the outcome.

Cohort study types

1. Prospective cohort study: The basic type of cohort study

2. Historical cohort study Or retrospective cohort study

3. Ambispective cohort study

Prospective cohort studies

Prospective cohort studies are conducted by making all observations on exposure and disease status after the onset of the investigation

Retrospective cohort studies

Retrospective cohort studies involve observations on exposure and disease status prior to the onset of the study

Cohort Study design and practice

·         Selection of Subjects

       The Exposed Group

       The Unexposed Group

       The Outcome Event

l    Follow up

l    Data Collection

Selection of exposed group

Exposure determining is based on a descriptive study and case-control study. The degree of exposure may differ depending on the goals of the study.

The investigator should identify an accessible population that is motivated to participate in the study and unlikely to discontinue participation

When the purpose of a cohort study is to investigate a community, such as in the Framingham Heart Study that community is the source of the unexposed persons.

Data Collection

Possible sources of data an exposure status include records, Interviews or Questionnaires, or Direct Measurements Made on Cohort Members.

Records

The available records include Occupational records, medical and pharmacy records, census records and so on

Direct Measurements

Many exposures of interest cannot be determined with any accuracy, or perhaps at all, for individual study subjects from either records or interviews, but can be determined by direct measurement.

Cumulative incidence rate

A number of new cases of disease occurring over a specified period of time in a population at risk at the beginning of the interval.

Cumulative Incidence

the probability (risk) of an individual developing the disease (outcome) during a specific period of time.

Incidence density

A number of new cases of disease occurring over a specified period of time in a population at risk throughout the interval. The probability (risk) of an individual developing the disease (outcome) during a specific period of time, using total person-time as the denominator.  One subject followed one year contributes one person-year (PY). 

Incidence density requires us to add up the period of time each individual was present in the population and was at risk of becoming a new case of the disease.

                                                                               

Incidence density characteristically uses as the denominator person-years at risk.  (Time period can be person-months, days, or even hours, depending on the disease process being studied.)

Types of epidemiological studies: Descriptive study

Descriptive study includes activities relates to characterizing the distribution of disease within a population. Descriptive studies can yield valuable information about a population's health
status, and they can be used to measure risks and generate hypotheses. It is also useful in health service evaluation and can be used periodically to determine whether a particular service is improving

The type of descriptive study

• Case reports
• Case series
• Cross sectional studies
• Ecologic studies

Case reports and series

Case report: describes an observation in a single patient.

ª  “I had a patient with a cold who drank lots of orange juice and got better.  Therefore, orange juice may cure colds.”

Case series: same thing as a case report, only with more people in it.

ª  “I had 10 patients with a cold who drank orange juice….”

Cross sectional studies

A cross-sectional study is a descriptive study in which disease and exposure status are measured simultaneously in a given population. Also called a “survey” or “prevalence” study Cross-sectional studies can be thought of as providing a "snapshot" of the frequency and characteristics of a disease in a population at a particular point in time. This type of data can be used to assess the prevalence of acute or chronic conditions in a population.

Research aim of prevalence survey

• To describe distribution of disease
• To discovery clue of pathogenesis
• Be used in secondary prevention
• To evaluate prevention and cure effect
• Surveillance of disease
• Health demand, health project and health policy decision

Describe the distribution of disease or health status by person, place and time, then analyze that which factors are relate to the disease or health status.

Secondary prevention seeks to minimize adverse outcomes of disease through early detection, even before symptoms develop and care is sought. Mammography for early detection of breast cancer in asymptomatic women is an example.

An occupational physician planning a coronary prevention program might wish to know the prevalence of different risk factors in the workforce under his care so that he could tailor his intervention accordingly.