Probability Sampling, defines and describes how to select commonly used samples

Sampling, presents key sampling concepts and terms; discusses types of random sampling; describes how to deal with sampling problems; and provides answers to some frequently asked questions about sampling

Sampling Guide, provides a guide to selection of samples; includes how to define measurement objectives for a survey; examines how sample size can be estimated; looks at ways of selecting samples; and discusses how to analyze the data collected

Sampling in Research, discusses general issues related to sampling for research purposes; identifies dangers in sampling and to minimize them, and provides guides for deciding on the size of a sample

Nonprobability sampling

Although we earlier advised using only probability sampling, sometimes nonprobability methods may be warranted. If the purpose of the research is to explore some topic, for example, to identify key concepts or to test respondents' understanding of items to be used in a scale, nonprobability methods are appropriate. In such cases, the purpose is not to generalize, which requires a random sample, but to learn something for later use. Also, when a sampling frame does not exist and cluster sampling is impractical, nonprobability methods are the only way to obtain a sample.

If any of these conditions apply, you may want to consider using some variation of the following nonprobability samples. But remember, lacking random selection of the sampling elements, nonprobability samples have two serious limitations:

• There is no control or protection against bias introduced by the investigator in the selection of the sample, whether this is by conscious or unconscious action.
• Since the variation that inevitably occurs in sampling cannot be estimated, results from a nonprobability sample cannot be safely generalized beyond the specific sample that was used.

Still, there are occasions, when nonprobability sampling methods may have to be used. Four frequently used nonprobability sampling methods are:

• Convenience sampling;
• Quota sampling;
• Purposive sampling; and
• Network sampling.

Convenience samples

In convenience sampling, also called haphazard sampling, the investigator selects some number of persons or other sampling units because they are easily accessible. Some illustrations of convenience samples are shown in Box 8.2.

Box 8.2. Illustrations of convenience samples Students found on the street near your university Persons selected by the investigator from those in a market on Monday morning The first 20 persons encountered at a bus stop Persons who respond to an invitation to comment on a television show All the mothers at the local well baby clinic on a Thursday morning