Study Design in Epidemiology

We almost always have to deal with limited and varying information. Its very important that how we draw random samples from the population. Let's first learn some key words:

Target Population:
The target population refers to the population to which we would like to apply our estimates and inferences regarding the relationship between disease and exposure.

Study Population:
The study population refers to a convenient subgroup of the population for which appropriate sampling frames are available and we are able to sample.

Target population vs study population

Differences between target population and study population introduce selection bias in our results if the study population is not representative of the target population with regard to the disease exposure relationship of concern.
[This does not necessarily require that the study population is representative of all aspects of the target population.]

Points to be considered in choosing the study design:
A good research design should perform the following function:

1. Enable a comparison of variable (such as disease frequency) between two or more groups at one point in time or in some cases, between one group before and after receiving and intervention or being exposed to a risk factor. [Intervention is something like a risk factor]
2.  Allow the comparison to be quantified either in absolute terms (as with risk difference or rate difference) or in relative terms (as with relative risk (RR) or odds ratio (OR).
3.  Permit the investigation to determine when the risk factor and the disease occurred, in order to determine the temporal sequence.
4.  Minimize biases, confounding and other problems that would complicate interpretation of the data.

Types of research design:

1. Observational designs:
   
    
   1. Population based studies (cross sectional studies)
   2. Cohort studies (exposure based sampling
   3. Case- control studies (Disease-based sampling)



2. Experimental designs:
   
   1. Randomized control clinical trials (RCCT)
   2. Randomized controlled field trials( RCFT)