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14 - Probabilistic Methods: Multivariate Statistics III: Techniques Free of Linear Assumptions

This chapter discusses nonparametric multivariate techniques. Time series analysis, which appears to follow the pattern of other least squares procedures, in fact presents special problems. Demographers attempting to project population shifts have long exploited time-series methods originally developed by economists for the analysis of long-term trend and periodic (cyclic and seasonal) fluctuations in economic indicators. Increasingly, social scientists interested in such questions as changes in voting patterns or religious affiliation, or other attitudinal shifts over time are making use of these techniques. To use this linear model and to apply the least-squares machinery, it is necessary to make three assumptions concerning the error terms: (1) the error term has mean zero, (2) the error term has constant variance independent of overall observations, and (3) the error terms corresponding to different points in time are not correlated. If a study requires precise decomposition of time series and examination of periodicities, one may wish to take advantage of spectral analysis. Its strength lies in its potential for providing better forecasts and its avoidance of the inaccuracies that arise from autocorrelation. However, the methods are untested in that relatively few studies exist to examine interpretations.

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8 - The Study Population: Sampling

This chapter provides an overview on sampling. The first step in a discussion of sampling is the identification of the target population or universe from which a sample is to be drawn. A population is the totality of all the cases, called population elements, or units, that meet some designated set of specifications. When one population is included in another, the former is called a subpopulation or a stratum. It is essential that the population units be clearly definable. Identifying the population properly is a difficult problem in itself. In any study, the choice of sample determines the generalizability of the results. To be useful, a sample must satisfy three criteria: (1) the sample must represent the population, (2) the sampling procedure must be efficient and economical, and (3) the estimates of population characteristics obtained from the sample must be precise and testable for reliability. The basic distinction in sampling is between probability and nonprobability samples. A probability sample is one in which the probability that any element of the population is included can be specified; in the simplest case, each element is equiprobable, but this is not necessary. It is only necessary that the probability of inclusion be knowable. In contrast, the probability of any element's inclusion in a nonprobability sample is unknown. The chapter also discusses the advantages and disadvantages of sampling.

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