<P> Time spent in making the sampled population and population of concern precise is often well spent, because it raises many issues, ambiguities and questions that would otherwise have been overlooked at this stage . </P> <P> In the most straightforward case, such as the sampling of a batch of material from production (acceptance sampling by lots), it would be most desirable to identify and measure every single item in the population and to include any one of them in our sample . However, in the more general case this is not usually possible or practical . There is no way to identify all rats in the set of all rats . Where voting is not compulsory, there is no way to identify which people will actually vote at a forthcoming election (in advance of the election). These imprecise populations are not amenable to sampling in any of the ways below and to which we could apply statistical theory . </P> <P> As a remedy, we seek a sampling frame which has the property that we can identify every single element and include any in our sample . The most straightforward type of frame is a list of elements of the population (preferably the entire population) with appropriate contact information . For example, in an opinion poll, possible sampling frames include an electoral register and a telephone directory . </P> <P> A probability sample is a sample in which every unit in the population has a chance (greater than zero) of being selected in the sample, and this probability can be accurately determined . The combination of these traits makes it possible to produce unbiased estimates of population totals, by weighting sampled units according to their probability of selection . </P>

True list of elements in a population is called what