Index

An index is a measure of an abstract theoretical CONSTRUCT in which two or more indicators of the construct are combined to form a single summary score. In this regard, a SCALE is a type of index, and, indeed, the two terms are sometimes used interchangeably. But whereas scales arrange individuals on the basis of patterns of attributes in the data, an index is simply an additive composite of several indicators, called items. Thus, a scale takes advantage of any intensity structure that may exist among the attributes in the data, whereas an index simply assumes that all the items reflect the underlying construct equally, and therefore, the construct can be represented by summing the person's score on the individual items.

[Page 486]

Indexes are widely used by government agencies and in the social sciences, much more so than scales. The consumer price index, for example, measures prices in various areas of the economy, such as homes, cars, real estate, consumer goods, and so forth. All of these entities contribute to the average prices that consumers pay for goods and services and therefore are included in the index. This example indicates why an index is generally preferred over a single indicator. Thus, the price of gasoline might spike at a particular time, but the price of other entities may be stable. It would be inaccurate and misleading to base the consumer prices just on the increasing cost of gasoline when the other items remained unchanged.

Furthermore, an index may not be unidimensional. For example, political scientists use a 7-point index to measure party identification. This index is arranged along a continuum as follows: strong Democrat, weak Democrat, independent leaning toward Democrat, independent, independent leaning toward Republican, weak Republican, strong Republican. One can easily see that this index consists of two separate dimensions. The first is a directional dimension, from Democrat through independent to Republican. However, there also exists an intensity dimension, going from strong through weak, independent, weak, and back to strong.

Frequently, governments use indexes of official statistics. For example, the consumer price index is used as a measure of the level of prices that consumers pay, and the FBI's crime index is the sum of the seven so-called index crime rates and is used as an overall indicator of crime in the United States.