Skin discoloration can be due to changes in melanin. Melanin is a skin pigment that gives color to the skin. Changes in melanin can also cause skin to be patchy.
Probability sampling, or random sampling, is a sampling technique in which the probability of getting any particular sample may be calculated.Nonprobability sampling does not meet this criterion and, as any methodological decision, should adjust to the research question that one envisages to answer.ampling is the use of a subset of the population to represent the whole population or to inform about (social) processes that are meaningful beyond the particular cases, individuals or sites studied.
A purposive sample, also commonly called a judgmental sample, is one that is selected based on the knowledge of a population and the purpose of the study. The subjects are selected because of some characteristic. References Babbie, E. (2001). The Practice of Social Research: 9th Edition.