Parameters are characteristics of populations-they describe the population. In research design, one is often interested in estimating certain parameters of a defined population or testing hypotheses on them. A clear understanding of the objectives of the study will influence the parameter(s) of importance. Consider the situation of environmental pollution in a certain city or region. Suppose that a measure of environmental pollution is the level of carbon monoxide in the air. A parameter could be the mean level of carbon monoxide per specified unit volume. This entry describes the types and categories of parameters; discusses the use of parameters in probability distributions, estimation, and hypothesis testing; and suggests a focus for future research.