As for a simple random sampling, it is a method which
executed in a manner that every element of the population
has equal probability of being included in the sample and a
complete frame including units of population is needed. In
addition, simple random sampling gives unbiased estimates
of population means and variances estimates .Thus, when
we do not select our sample randomly out of the population
of interest, our sampling results may be biased. Hence, the
necessity of simple random sampling arises.