temperature-humidity index: see humidity.

index, cephalic: see cephalic index.

index number, in econometrics, a figure reflecting a change in value or quantity as compared with a standard or base. The base usually equals 100 and the index number is usually expressed as a percentage. For example, if a commodity cost twice as much in 1970 as it did in 1960, its index number would be 200 relative to 1960. Index numbers are used especially to compare business activity, the cost of living, and employment; one of the most influential indexes in the United States is the Consumer Price Index (see under cost of living). Index numbers enable economists to reduce unwieldy business data into easily understood terms.

index, of a book or periodical, a list, nearly always alphabetical, of the topics treated. This list is usually at the back of a book, and the table of contents is in the front. The index seeks to direct the reader to all names and subjects on which the book has information. The subject, with the number of the page on which related information is to be found, is called the entry. In an index to a periodical the entries are less specific, referring usually to an article as a whole rather than to every subject touched upon in each article. Indexing requires experience and skill, since it is necessary not only to grasp the meaning of the author but to phrase that meaning clearly and in such a way as to place it alphabetically where the reader is likely to look first. Books written to give information are of little value unless properly indexed. Indexes to books were made long before the invention of printing. In the 16th cent. the term index began to be commonly applied to such a list; until the 17th cent. the index was rarely alphabetical. Diderot's famous Encyclopédie (1751-1772) had an alphabetical index. In 1848 in the United States a general index to the most widely circulated periodicals of the time was issued by William Frederick Poole. Poole's Index, later compiled cooperatively, continued until 1907, when it was superseded by the Readers' Guide to Periodical Literature. There are special indexes in various fields of knowledge, e.g., law, medicine, art, education, engineering, industrial arts, agriculture. Newspaper indexes include those to the London Times (from 1906) and the New York Times (from 1851). Indexes are increasingly being compiled by computer, and published as on-line databases and in CD-ROM format. The H. W. Wilson and R. R. Bowker companies are noted for special annual indexes, particularly the Readers' Guide to Periodical Literature, the Cumulative Book Index, and Ulrich's International Periodicals Directory. Indexes to illustrations, to artifacts, to formulas, and to various collections of materials are common. Some are alphabetical; others may be by number, color, or some other scheme. The catalog of the books in a library is sometimes known as an index.

cranial index: see cephalic index.

color index, in astronomy, difference in an object's brightness as recorded between any two well-defined bands of the electromagnetic spectrum by using optical filters of different colors. If blue and red filters are used, then the color index B-R would be zero for a white star (spectral class A0). The color index is positive for stars redder than a white star and negative for stars bluer than a white star. In effect, measuring the color index is equivalent to measuring the difference between the amount of blue light and red light that the star radiates. Although a star's true color might be changed as its light travels through clouds in interstellar space, the color index is an important indicator of the temperature of a star, gas cloud, or galaxy.

cephalic index [Gr. kephale=head], ratio of the breadth of the head to its length. Expressed as a percental number, it provides the simplest description of the geometric relation of two dimensions. The index is obtained by dividing the maximum width of the cranium by its maximum length and multiplying by 100. In anthropometry, the cephalic index has been the favored measurement. A cephalic index of 80 or more is called brachycephalic or broad; a measurement between 75 and 80 is mesaticephalic; below 75 is considered dolicocephalic or long. The cranial index is the same ratio taken on a skull.

Index, in the Roman Catholic Church, list of publications forbidden to be read, called Index librorum prohibitorum [list of forbidden books]. This censorship was exercised by the Holy See. Catholics are forbidden, as a natural part of ethics, to read anything they know may endanger their faith or moral life; the Index was a partial guide to such literature. Since it was made up only from decisions referred for judgment on specific works, there was no consistency of inclusion; the failure of a book to appear in it implied nothing. The last edition of the Index was published in 1948. In 1966 the Congregation for the Doctrine of the Faith (formerly, the Holy Office) announced that the Index and its related penalties of excommunication would no longer have the force of law in the church.

Consumer Price Index: see cost of living.

Measure of living costs based on changes in retail prices. Consumer price indexes are widely used to measure changes in the cost of maintaining a given standard of living. The goods and services commonly purchased by the population covered are priced periodically, and their prices are combined in proportion to their relative importance. This set of prices is compared with the initial set of prices collected in the base year to determine the percentage increase or decrease. The population covered may be restricted to wage and salary earners or to city dwellers, and special indexes may be used for special population groups (e.g., retirees). Such indexes do not take into account shifts over time in what the population buys; when modified to take subjective preferences into account, they are called constant-utility indexes. Consumer price indexes are available for more than 100 countries.

Learn more about consumer price index (CPI) with a free trial on Britannica.com.

Measure of change in a set of prices, consisting of a series of numbers arranged so that a comparison of the values for any two periods or places will show the change in prices between periods or the difference in prices between places. Price indexes were first developed to measure changes in the cost of living in order to determine the wage increases necessary to maintain a constant standard of living. There are two basic types. Laspeyres-type indexes define a market basket of goods in a base period, then use the prices for those goods to examine change over space and time. In its simplest form, this is simply the ratio of what those goods cost today to what they cost in the base period. The two most familiar indexes of this type are the consumer price index (CPI) and the producer price index (PPI). The CPI measures changes in retail prices in such component parts as food, clothing, and shelter. The PPI (formerly called the wholesale price index) measures changes in the prices charged by manufacturers and wholesalers. Paasche-type indexes define a market basket of goods in the current period, then use the prices of those goods from past periods. The most familiar index of this type is the GDP deflator, used in the U.S. in the national income accounting to differentiate amounts in constant dollars from those in current dollars.

Learn more about price index with a free trial on Britannica.com.

Comparison of price levels over time. In fiscal policy, indexation is used as a means of offsetting the effect of inflation or deflation on social security payments and taxes by measuring the real value of money from a fixed point of reference, usually a price index. Without indexing, recipients of social security benefits, for example, would suffer during times of inflation if their benefits remained at a fixed rate. Indexation is used in some countries to offset “bracket creep,” which occurs in any progressive tax system when inflation pushes taxpayers into higher tax brackets. Indexation may also refer to the linking of wage rates and financial instruments to a price index.

Learn more about indexation with a free trial on Britannica.com.

In igneous petrology, the sum of the volume percentages of the coloured, or dark, minerals in the rock. The most common light-coloured minerals are feldspars, feldspathoids, and silica or quartz; abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals.

Learn more about colour index with a free trial on Britannica.com.