sampling

sampling
1. (time quantization) A process by which the value of an analog, or continuous, signal is “examined” at discrete fixed intervals of time. The resulting sampled value will normally be held constant until the next sampling instant, and may be converted into a digital form using an A/D converter for subsequent processing by a computer.

The rate at which a given analog signal is sampled must be a certain minimum value, dependent upon the bandwidth of the analog signal; this ensures that none of the information in the analog signal is lost. The sampling rate may also affect the stability of an analog system if the system is to be controlled by a computer. See also Nyquist's criterion.

2. The act of selecting items for study in such a way that the measurements made on the items in the sample will provide information about similar items not in the sample. Items can be people, machines, periods of time, fields of corn, games of chance, or whatever is being studied. Sample size is the number of items included in the sample. If the variance of the measurement (see measures of variation) is approximately known, the variance of its mean in a sample is the population variance divided by the sample size. This formula can then be used to indicate an appropriate sample size.

A population is a complete set of items about which information is required. It must be defined before selecting the sample or results may be ill-defined. The sample is the basis for inference about probability distributions of measurements on the population. Problems of sampling include avoidance of bias and selection of enough samples to ensure adequate precision.

Random sampling is the process that results in each item having the same probability of inclusion in the sample. Items may be selected with the aid of tables of random numbers or with mechanical devices such as cards or coins.

Systematic sampling selects items in some regular manner. It is valid when the order in which items are encountered is irrelevant to the question under study, but can be an unintentional source of bias.