Well do the the fact when one is doing a testing the sample

Well do the the fact when one is doing a testing, the sample needs to be part of the population. If the sample did not represent the population one would not be able to conclude anything due to the sample is not part of the population. When using probability sample techniques one has to make sure that the sample of part of the population. Picking out the sample needs to be unbiased. By being unbiased, one needs to choose the sample at random. For example if the survey in a paper about women and how they much they weigh. The people who respond to the survey will only the people reading the paper. So not only does the sample need to be random the technique needs to be done by experiment by using SRS. (Simple Random Selection).

Solution

Yes, the testing sample must be a part of the population. They must be selected in random.

As given in the example, if we consider a online survey then people filling their weight details will be working womens who\'s age lies between 20-50, but it will not give the exact picture since the smaller baby infants and the old people data will not be collected in this case.

In order to have better approximation, Simple Random Selection (Each individual is chosen randomly and entirely by chance, such that each individual has the same probability of being chosen at any stage during the sampling process, and each subset of k individuals has the same probability of being chosen for the sample as any other subset of kindividuals)

Since it will give better approximation to the confined probability result