Age data are usually available in the form of single years, that is, the number of persons in the population of age 13, 14, 15 etc. For demographic analysis, however, age data are seldom used in this form.

It is customary to classify them in five-year age groups, such as 0-4, 5-9, 10-14, 15- 19, 20-24 and so on. The age data thus classified in mutually exclusive age groups are used for an analysis of the age structure of any population, and are also useful for a wide variety of analytical Purposes.

The age structure of any population is most commonly studied With the help of a simple mathematical measure like the per cent distribution and an equally simple measure like the age pyramid, the other measures used for an analysis of the age structure are he average age (mean, median, mode), and a few other indices based on the distribution of persons in various age groups.

The simplest measure is the percentage distribution of the population based on the absolute numbers in various five-year age groups, and indicates the number of persons in an age group if the total number of persons considered is 100.

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This measure is helpful in forming an idea about the age structure of any population, and is used for describing it. It is also used to compare the age structures of two or more populations at a point of time or to compare the age structure of the same population at different points of time.

The age-sex histograms, known as the age-sex pyramids, may be used for precisely the same purposes. Table 6.7 is an illustration of the percentage age distribution of the population of India as in 1991, which is presented in five-year age groups both in terms of absolute numbers and percentages.

Illustrates how the age distribution of two countries may be compared with the help of percentage distribution by presenting the age distribution of the population of India (1991) and of the United States (1995) in broad age groups.

It is evident from Table 6.8 that the age distribution of the Indian population differs a great deal from that of the American population. The percentage of Indians in the age group 0-4 is a little less than twice that of the Americans.

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On the other hand, the percentage of Americans above age 65 is almost three times that of Indians. From this information, it is clear that there are more children in the Indian population and more elderly people in the American population.

Another measure to study the structure of the population is the dependency ratio. This measure indicates the number of dependents per 100 workers, and may be computed on the basis of three broad age groups, that is, below 15, between 15-59 or 15-64, and 60 or 65 and above.

The population in the age group 15-59 or 15-64 is considered to be the working population; that below 15 as the young dependents, and that above either 60 or 65 is considered to be old dependents.

The dependency ratio is not a completely accurate measure for assessing the dependency burden, for not all persons in the working age are employed, nor are all those in the dependent age economic dependents. This measure, however, gives us a broad idea of economic dependency in any population and is, therefore, widely used.