Summary 2 : Sampling

·         A sample in a research study is a group on which information is obtained. The larger group to which one hopes to apply the result is called the population.

·         However, a group may be both a sample in one context and a population in another context. Eg : All state university students who own the mobiles constitute the population of mobile owners at State, yet they also constitute a sample of all automobile owners at state universities across the United States.

·         Random Sampling : After making a decision to sample, researchers try hard, in most instances, to obtain a sample that is representative of the population  of interest -  that means they prefer random sampling. There are three types of this which are: simple random sampling, stratified random sampling, and cluster sampling. A less common method is two-stage random sampling.

·         Simple Random Sampling is the purest form of probability sampling. Each member of the population has an equal and known chance of being selected. When there are very large populations, it is often difficult or impossible to identify every member of the population, so the pool of available subjects becomes biased.

·         Stratified random sampling is commonly used probability method that is superior to random sampling because it reduces sampling error. A stratum is a subset of the population that share at least one common characteristic. Examples of stratums might be males and females, or managers and non-managers. The researcher first identifies the relevant stratums and their actual representation in the population. Random sampling is then used to select a sufficient number of subjects from each stratum. "Sufficient" refers to a sample size large enough for us to be reasonably confident that the stratum represents the population. Stratified sampling is often used when one or more of the stratums in the population have a low incidence relative to the other stratums.

·         Cluster random sampling is the selection of groups, or clusters of subjects rather than individuals.

·         There are three types of non-random sampling which are systematic sampling, convenience sampling, and purposive sampling.

Reference:

Fraenkel J.R., & Wallen N.M.(2010). How to Design and Evaluate Research in Education,7,90-105.