Standardised birth rate tells real trend

THE crude birth rate in Hong Kong showed a drop by 37 per cent over the past 20 years, from 19.5 live births per 1,000 persons in 1972 to 12.3 in 1992.

This is not the true ''underlying'' birth trend.

To reveal the birth trend in a population, the crude birth rate is not a good indicator. This is because it relates the number of births to the entire population, including both men and women. Any change in the size of the population would affect the rate.

For instance, an outflow of male population decreases the population size, leading to a higher birth rate, while actually the birth trend may not have changed.

The more crucial factor is the age distribution of the female population. For a population with more women in the ages of child-bearing between 15 and 49, it will have a higher birth rate but despite that its birth trend may not have changed, or vice versa.

In the study of fertility, demographers use the age-specific fertility rates. For a given age group (within 15 to 49), this rate represents the number of live births per 1,000 women in that age group.

For a general view of the birth trend in a population, the age-specific birth rates give too much information. To compromise between the crude birth rate and the detailed rate, demographers apply an age-adjusted or ''standardised'' rate, using a single figure for a population but making allowance for changes in its age distribution.