Multilingual Demographic Dictionary, second unified edition, English volume

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160

Sampling procedures ¹ are used to obtain information about a population from part of the population only instead of having to study every person (110-2). The part of the population studied is called a sample ². A population is a collection of elements ³ which are the object of the investigation. A sampling unit ⁴may be an element or a group of elements of the population and is used for selecting samples. In demographic samples the elements are generally individuals (110-2), families (112-1) or households (110-3), and sampling units may be individuals, households, blocks of houses, municipalities or areas. The sample will consist a number of sampling units selected in accordance with a sampling scheme ⁵ or sampling plan ⁵.

161

A sample whose elements are selected by a chance process is called a probability sample ¹. If a complete list of sampling units is available this is called a sampling frame ³. In simple random sampling ⁴ a proportion of sampling units is selected from the frame at random ². This proportion is called the sampling fraction ⁵. Systematic samples ⁶ are taken from a frame in which the sampling units are consecutively numbered. The sample is drawn by taking the n^th, (n + s)^th, (n + 2s)^th..., etc. unit, where n is not larger than s and is selected at random. In cluster sampling ⁷ population elements are not drawn individually, but in groups which are called clusters ⁸.

• 2. random — randomness n. — randomize v.

162

In stratified random sampling ¹ the population is divided into a number of strata ² which are in some sense more homogeneous than the population as a whole with respect to the characteristics studied, and a simple random sample (cf. 161-4) is drawn in each stratum. Variable sampling fractions (161-5) may be used in different strata. Multi-stage sampling ³ is a method where the selection of the sample is carried out in several stages. A sample of primary sampling units (160-4) is first selected and each of these units is then regarded as a population from which a sub-sample ⁴ is selected, and the process may be repeated. Where there is no good sampling frame, a sample of areas delimited on a map may be selected: this procedure is called area sampling ⁵.

163

In probability sampling (161-1), chance methods are used to obtain a representative sample ¹, i. e., a sample which is a relatively faithful reflection of the population with respect to the character or characters studied. In quota sampling ², on the other hand, the sample is purposely selected so as to reflect the population in certain characteristics, and each interviewer (204-2) is given a quota ³ of different types of sampling units which are to be included in his sample. Within the limits of the quota the interviewer is free to select the sampling units.

164

A population parameter ¹ is a numerical value that characterizes a population. Statistical estimation ² is the name given to the procedure by which the values of such parameters can be estimated from the sample. Such estimates are subject to sampling errors ³ and a measure of the magnitude of the sampling error is generally given by the standard error ⁴. Sometimes a confidence interval ⁵ is associated with an estimate to show the limits within which the estimated quantity may be expected to lie. A difference between two values is called a significant difference ⁶ when the probability that it is due to chance is less than a given value which is called the level of significance ⁷. Thus a difference will be significant at the 5 per cent level if the probability that it could have arisen by chance is less than 0.05.