A cohort study or panel study is a form of longitudinal study (a type of observational study) used in medicine, social science, actuarial science, business analytics, and ecology. For instance in medicine, it is an analysis ofrisk factors and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction. It is one type of clinical study design and should be compared with across-sectional study. Cohort studies are largely about the life histories of segments of populations, and the individual people who constitute these segments.^[1][2]

A cohort is a group of people who share a common characteristic or experience within a defined period (e.g., are born, are exposed to a drug or vaccine or pollutant, or undergo a certain medical procedure). Thus a group of people who were born on a day or in a particular period, say 1948, form a birth cohort. The comparison group may be the general population from which the cohort is drawn, or it may be another cohort of persons thought to have had little or no exposure to the substance under investigation, but otherwise similar. Alternatively, subgroups within the cohort may be compared with each other.

Randomized controlled trials (RCTs) are a superior methodology in the hierarchy of evidence in therapy, because they limit the potential for any biases by randomly assigning one patient pool to an intervention and another patient pool to non-intervention (or placebo). This minimizes the chance that the incidence of confounding (particularly unknown confounding) variables will differ between the two groups. However, it is important to note that RCTs may not be suitable in all cases and other methodologies could be much more suitable to investigate the study's objective(s).

http://en.wikipedia.org/wiki/Cohort_study

Cohort Studies

Cohort is an ancient Roman military unit of 300-600 men, a group of soldiers marching forward in battle. Cohort studies are longitudinal, prospective studies, forward looking studies or incidence studies. They start with people free of disease. They assesses exposure at “baseline” and disease status at “follow-up”.

2 x 2 Table

2 x 2 table is used to summarize counts of disease and exposure in order to perform calculations of association.

Here,
a = number who are exposed and have the outcome
b = number who are exposed and do not have the outcome
c = number who are not exposed and have the outcome
d = number who are not exposed and do not have the outcome
a + b = total number who are exposed
c + d = total number who are not exposed
a + c = total number who have the outcome
b + d = total number who do not have the outcome
a + b + c + d = total study population

Relative Risk (RR)

It is the “ratio of incidence of disease among exposed to incidence of disease among non- exposed”
Incidence among  exposed
Relative Risk =       ———————————-
Incidence among not exposed

The relative risk is the risk of disease in the exposed group divided by the risk of disease in the non-exposed group. RR is the measure used with cohort studies

Cohort Study Examples

•    Study to determine if smokers have a higher risk of lung cancer
•    Study to determine if children who receive influenza vaccination miss fewer days of school
•    Study to determine if the Russian Salad was the cause of a food borne illness outbreak

Advantages of Cohort Studies

1.    Temporality:  Exposure precedes outcome because the cohort is disease free at baseline
2.    Efficient for studying rare exposures
3.    May be used to study multiple outcomes
4.    Allows for calculation of incidence of diseases in exposed and unexposed individuals
5.    Minimizes recall bias

Disadvantages of Cohort Studies

1.    Tend to be expensive (large sample size) and time consuming (long follow-up period)
2.    Loss to follow-up: When multiple outcomes or specific disease incidence is the outcome of interest, bias can be a serious problem
3.    Inefficient to study rare diseases

Framingham Study Design

Framingham, Massachusetts population was 28,000. The study started in 1948. The study design called for a random sample of 6,500. Enrollment questionnaire from targeted age range 30-59 years was filled. Only those were included with no clinical evidence of atherosclerotic cardiovascular disease. Cohort was re-examined every two years.

Exposures included:
•    Smoking
•    Alcohol use
•    Obesity
•    Elevated blood pressure
•    Elevated cholesterol levels
•    Low levels of physical activity, etc.

Following hypotheses were generated:
•    Persons with hypertension develop CHD at a greater rate than those who are normotensive
•    Elevated blood cholesterol levels are associated with an increased risk of CHD
•    Tobacco smoking and habitual use of alcohol are associated with an increased incidence of CHD
•    Increased physical activity is associated with a decrease in development of CHD
•    An increase in body weight predisposes a person to CHD

Types of Cohort Studies

1.    Concurrent Cohort Study (prospective or longitudinal)
2.    Retrospective Cohort Study (historical cohort or non-concurrent prospective study)

Both designs are identical, comparing exposed and non-exposed populations. The only difference is calendar time.

Concurrent Cohort Study

In concurrent cohort study, the investigator identifies original study population at the beginning of the study. The individuals are followed prospectively through time until disease develops or does not develop. Disadvantages of this type of study include:
1.    Requires long follow-up time (years)
2.    Expensive
3.    Age of investigator may be a limiting factor

Retrospective Cohort Study

In this case, the cohort is defined from historical data and followed up for disease up to the present time. It can telescope the frame of calendar time for the study and obtain results sooner. Disadvantage of this type of study is that quality depends on the historical data that is available – both to define exposure and to identify the outcome.

Assessment of Exposure

Techniques used to measure exposure include questionnaires (age, smoking habits), laboratory tests (cholesterol, hemoglobin), physical measurements (height, weight, blood pressure) and various special procedures (EKG, x-rays). Quantifying exposures includes information such as date of onset, frequency of exposure and duration and intensity of exposure.

Exercise

How would you design a cohort study of the association between preterm delivery and cigarette smoking?

Results
1.    An exposed and a non-exposed group would first be identified e.g.) women presenting to a local county health department for prenatal care would be classified by smoking status – smokers and non-smokers.
2.    These women would be followed to determine whether or not preterm delivery occurred.
3.    The rates of the preterm delivery would be compared among the smokers and non-smokers.
4.    The objective of the cohort study is to test a hypothesis regarding the causation of disease.
5.    The group of persons to be studied (cohort) are defined in terms of characteristics manifest prior to appearance of the disease being investigated.
6.    The defined study groups are observed over a period of time to determine and compare the frequency of disease among them.

http://howmed.net/community-medicine/cohort-studies/