It should be remembered that entire population is not going to be studied except the relevant population .

For e.g. for cancer cervix studies, we choose the women in reproductive age, not female children and certainly not males.

Reference population is the one, which has to be get benefited after our descriptive study.

Thus the population at risk is better one to be studied rather than the total population while formulating hypothesis as the inferences after testing hypothesis cannot be generalized to all the components of whole population except to the specific and vulnerable/susceptible portion of it.