A time series is a sequence of observations, usually...

A time series is a sequence of observations, usually ordered in time, although in some cases the ordering may be according to another dimension. The feature of time series analysis which distinguishes it from other statistical analysis is the explicit recognition of the importance of the order in which the observations are made. While in many problems the observations are statistically independent, in time series successive observations may be dependent, and the dependence may depend on the positions in the sequence. The nature of a series and the structure of its generating process also may involve in other ways the sequence in which the observations are taken.

Theodore Wilbur Anderson

T. W. Anderson. The Statistical Analysis of Time Series, (1971/2011), p. 1. Introduction; Cited in: American Sociological Association (1974), Sociological Methodology, p. 310

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