Few statistical series are considered by themselves; rather one series is compared with others, usually interrelated, in order that significant relationships may be brought out, either as sequences or as maladjustments. To attain this objective it is often necessary to put the time series, as originally reported, through various statistical processes.

The easiest and simplest way to handle a series of statistical data may be to use the figures reported. If only one series is being considered, it is not of much importance whether the figures represent absolute units, such as dollars, tons, gallons, or pieces, or are expressed as "relatives," that is, percentages of some base period. Comparisons among the absolute figures may be made just as readily as if pains were taken to convert each item of the series into a percentage of some common base.

Whether or not a series of business data is converted from the absolute to the relative form, it may be an indicator, or "index," of an economic condition. "Index numbers" were formerly considered as composites of relatives and were used almost exclusively to measure price changes. But nowadays there is no such restriction. Index numbers may represent stocks, production, sales, or other business measurements. Furthermore, there are many series which may be called index numbers although they are reported in absolute terms, such as dollars. Both the Bradstreet and the Dun commodity-price indexes are of this type and illustrate the fact that an index number does not have to be expressed in relative terms.

Neither does an index number necessarily imply a composite of a group of items. The price of middling upland spot cotton at New York is a standard raw-cotton price measurement. In the form of a uniform series this price becomes an indicator of the trend of raw-cotton prices and truly constitutes an index, though it represents only one grade of one commodity.

Along with the increased use of economic series there has developed a broader meaning of the word "index," until now practically any series may be considered an indicator of some specific economic condition and therefore an index. Yet to the term "index number" still clings a restricted meaning. To most of us it refers to those composites, usually in relative form, and frequently weighted, such as the wholesale commodity-price index of the Bureau of Labor Statistics