Given a certain population there are various ways in which a sample may be drawn from it. The chart below illustrates this point:
Figure 1
In Judgemental Sampling the sample is selected according to the judgement of the investigators or experts. Hence, there is a certain degree of subjectivity in the selection.
For example, the population under study may be people living in rented accommodation in Hyderabad. The investigator may choose as a sample, a certain number of his friends in Hyderabad living in rented accommodation. He may exercise all care in attempting to make the sample representative of the population.
But we cannot use Statistical Theory to find out how good the results of such a sampling are. Hence, we will concern ourselves mainly with Random Sampling, where every element of the population has a known chance of being included in the sample.