Probability Sampling:

Probability sampling involves the use  of  random selection process  to  select a sample  from members  or  elements of a population. The goal of probability sampling is to examine the representative elements of the population. Random selection  is  scientific, systematic process  in which each population element has a known equal chance or probability of being selected for the sample. Selection are independent  to each other and investigators bias does not enter the selection of the sample. If sample  is not  randomly selected, the ability to generalize the findings of  the study are greatly reduced.