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.