selecting on the basis of convenience.
every combination of a given size has an equal chance of being chosen.
dividing the population into clusters, typically on the basis of geography, and taking a sample of the clusters.
asking individuals studied to provide references to others.
sampling subunits within sampled units.
dividing the population into groups on the basis of some characteristic and then sampling each group.
selecting fixed numbers of units in each of a number of categories.
choosing every nth item from a list, beginning at a random point.
stopping when enough data have been collected.
selecting sequentially on the basis of earlier observations.