Statistical Theory

The field of statistics uses numerical information obtained from samples to draw inferences about populations. A population is a well-defined set of individuals, events, or objects. A sample is a selection of individuals, events, or objects taken from a well-defined population. A sample is generally taken from a population with each individual, event, or object being independent and having an equally likely chance of selection. The sample average is an example of a random sample estimate of a population value, i.e., population mean. Population characteristics or parameters are inferred from sample estimates, which are called statistics. Examples of population parameters are population proportion, population mean, and population correlation. For example, a student wants to estimate the proportion of teachers in the state who are in favor of year-round school. The student might make the estimate on the basis of information received from a random sample of 500 teachers in the population comprised of all teachers in the state. In another example, a biologist wants to estimate the proportion of tree seeds that will germinate. The biologist plants 1,000 tree seeds and uses the germination rate to establish the rate for all seeds. In marketing research, the proportion of 1,000 randomly sampled consumers who buy one product rather than another helps advertising executives determine product appeal.