Sample 04
Sample 04 : (a) A selected subset of a population. A sample may be random or nonrandom and may be representative or nonrepresentative. Several types of sample can be distinguished, including the following: Cluster sample: Each unit selected is a group of persons (all persons in a city block, a family, etc). rather than an individual. Grab sample (Syn: sample of convenience): These ill_defined terms describe samples selected by easily employed but basically nonprobabilistic methods. "Man_inthe_street" surveys and a survey of blood pressure among volunteers who drop in at an examination booth are in this category. It is improper to generalize from the results of a survey based upon such a sample for there is no way of knowing what sorts of bias may have been operating. Probability (random) sample: All individuals have a known chance of selection. They may all have an equal chance of being selected, or, if a stratified sampling method is used, the rate at which individuals from several subsets are sampled can be varied so as to produce greater representation of some classes than of others. A probability sample is created by assigning an identity (label, number) to all individuals in the "universe" population, e.g., by arranging them in alphabetical order and numbering in sequence, or simply assigning a number to each, or by grouping according to area of residence and numbering the groups. The next step is to select individuals (or groups) for study by a procedure such as use of a table of random numbers (or comparable procedure) to ensure that the chance of selection is known. Simple random sample: In this elementary kind of sample each person has an equal chance of being selected out of the entire population. One way of carrying out thil procedure is to assign each person a number, starting with 1, 2, 3, and so on. The l numbers are selected at random, preferably from a table of random numbers, until the desired sample size is attained. Stratified random sample: This involves dividing the population into distinc subgroups according to some important characteristic, such as age or socioeconomi status, and selecting a random sample out of each subgroup. If the proportion o the sample drawn from each of the subgroups, or strata, is the same as the propor tion of the total population contained in each stratum (e.g., age group 40_59 con stitutes 20% of the population, and 20~o of the sample comes from this age stra sum), then all strata will be fairly represented with regard to numbers of person in the sample. Systematic sample: The procedure of selecting according to some simple, systemat rule, such as all persons whose names begin with specified alphabetical letters, bar on certain dates or located at specified points on a master list. A systematic sampl may lead to errors that invalidate generalizations. [Last, 1983: A Dictionary of Epidemiology]; (b) A small part of something designed to show the nature or quality of the whole. Exposurerelated measurements are usually samples of environmental or ambient media, exposures of a small subset of a population for a short time, or biological samples, all for the purpose of inferring the nature and quality of parameters important to evaluating exposure. [USEPA, 1992: GL for Exposure Assessment] [USEPA, 1997b: Exposure Factors Handbook]
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