Incidence rate can calculated by
The biggest advantage of carrying out a cohort study is that it helps in calculating the incidence rate which provides a direct measure of risk in terms relative risk. Incidence rate is also known as absolute risk.
Theliteral meaning of the term ‘cohort’ refers to a group that shares similar characteristics.
- Cohort studies are FORWARD LOOKING; look for the development of disease in a group of individuals (the cohort) free of the same at the beginning.
- The group is followed up over a period of time. During this period some persons will develop the disease under study while others will remain free of the disease (FOLLOW-UP STUDY).
- The characteristics (and exposure to disease causing factors) are compared between those who suffer from disease and those free from the disease.
- Thus, in cohort study, identified groups of populations who are free of the disease being studied and who are similar in all respects, except the specific exposure variable or characteristic whose effect is being related to the disease being studied. These groups are then followed up for the period of time that it takes for the disease to develop.
- A cohort study can either be prospective or retrospective but unless otherwise specified it is assumed to be prospective in design, but a case control study is always retrospective.
Case control study:
- An epidemiological study where a group of individuals with disease (CASE) are compared with a group of individuals who are not suffering from disease (CONTROLS) in terms of specific disease causing exposures.
- Since the starting point is a group of people who already have suffered from the disease, this is labeled as a retrospective study.(BACKWARD LOOKING STUDY)
- Advantages of case control studies
a. Relatively quick and easy to undertake.
b. Relatively cheap to undertake.
c. Only methoduseful in rare diseases.
d. Not enmeshed in problems of follow-up as the data is collected at one point in time.
e. Can be used to study the effect of many exposure variables on a single disease outcome.
A case control study on the other hand only provides an estimate of risk in the form of an odds ratio.
Prevalence can be calculated using a cross sectional study design.
Cross sectional studies:
- It examines the relationship between the disease and other characteristic of interest, as they exist in a defined population at a given time.
- Easy to carry out.
- If the population is followed for more than once it becomes longitudinal study.
- Multiple cross sectional studies may prove to be cost effective.
- They are commonly known as PREVAELANCE STUDY.
A longitudinal study also helps in calculating incidence if used synonymously as a cohort study.
a. A prospective study design is the classical cohort study design which provides the incidence rate of the disease or the outcome under study & hence also helps in directly computing the relative risk of the disease.
b. A cross sectional study provides the prevalence rate or the total number ( or proportion) of diseased persons in the population at a point of time or over a period of time.