In gathering data about a group of individuals or items, rather than conducting a full census, very often a sample is taken from a larger population in order to save time and resources. These samples can be classified into two major groups describing the way in which they were chosen: probability samples and nonprob-ability samples. Both types of samples are made up of a basic unit called an individual, observation, or elementary unit. These are the units whose characteristics are to be measured from a population. In probability samples, each member of the population has a known nonzero probability of being chosen into the sample. By a random process, elements are selected and receive a known probability of being included in the sample; this is ...