Time series

The object of this note is to provide an introduction to methods used in time-series analysis, together with a general appraisal of their usefulness. The treatment is purely expository and, of necessity, rather condensed. Nothing more than a broad survey of the subject is attempted; those interested in the technical details of theory and application should consult the references given at the end.The termtime seriesis applied to any series of observed values of some character when theorderof the items in the series, as well as their actual magnitudes, is of importance. Examples of such series are common in statistical practice, and the following may be cited in illustration: economic series such as weekly, monthly or annual production figures, prices or index numbers; series of results of routine inspection and testing of successive batches of manufactured product; continuous blood-pressure records; results of psychological fatigue tests expressed as the rate of working on some standard task in successive 5 sec. intervals. In all these examples the factor which determines the order of the observations istime. The study of ordered series of observations is entitled ‘Time Series’ because in a large proportion of applications time is the ordering factor. Nevertheless, it is not necessary that the series be ordered according to time.