The common sampling methods are random, judgmental, systematic, stratified, and cluster sampling. 

In random sampling, a sample is obtained by randomly selecting individuals from a population without preference, each with an equal chance of being selected.

On the contrary, judgmental sampling is a non-random process. Here, a sample is selected based on the available information about the population.

Systematic sampling is a method of selecting individuals from a population at regular intervals. For example, to generate a sample size of 4 individuals from a population of 12, the starting point is chosen randomly between one and the sampling interval. So, every 3rd member is selected to make a sample of 4 individuals.

In stratified sampling, the population is divided into subgroups or strata based on the characteristics shared by the individuals. Individuals are then selected proportionally from each subgroup to compose a final sample for analysis.

Likewise, cluster sampling also involves dividing the population into subgroups or clusters. A few clusters are then randomly selected for assessment.