AS at end-1992, Hongkong's population was 5.9 million, averaging 5,590 people per square kilometre of land area. This makes Hongkong one of the most densely populated places in the world. In comparison, Singapore and Tokyo have densities of about 4,300 and 5,500 people per sq km respectively. Population studies are crucial to both government administration and business planning. In these studies, statistical measures such as the median age of the population are often found useful. The size of a population is always changing due to the combined effects of birth, death and migration. Births and immigration contribute to increase, while deaths and emigration contribute to decreases. The crude birth rate relates the number of live births to the mid-year population in a specified year. This helps to show the birth trend in the population, the crude death rate, which is similarly defined, may be used to reflect the death trend. However, these alone are not sufficient to give a full picture as they are influenced by the population structure. For instance, a population with more women of child-bearing age will normally have a higher crude birth rate. Conversely, a population withmany old people will be expected to carry a higher death rate. For more in-depth study, demographers would use a method called ''standardisation'' to derive alternative rates to eliminate the effects of the age-sex composition of the population. A snapshot of the age-sex composition of a population can best be presented in the form of a population pyramid (see diagram). Population pyramid is a diagram consisting of horizontal bars stacking on top of each other. Each year gives the total size of population in a particular age group. The age groups are arranged in ascending order with the youngest group at the bottom. Each bar has a male and a female component. The male population total is shown on the left hand side of the pyramid and the female population total on the right. The pyramid can be scaled to show the total population in either numbers or percentages. How does the Hongkong population pyramid look like? During World War II, there were relatively fewer births, while deaths of younger children were more prevalent due to adverse conditions. This resulted in a smaller percentage of the population in the age group 45-49 in the 1991 pyramid, who were the survivors of those people born during the war. After the war, high birth rates were prevalent during the period 1947-1966, particularly in the 1950s. These are the so-called baby-boom years. In the meantime, there were substantial declines in the death rates of the newborn and young children. The result was a high percentage of the population aged between 25 and 44 in the 1991 pyramid. Compared with 1981, the lower part of the 1991 pyramid is narrower and the upper part wider. This is because there were proportionately less newborn babies in the past 10 years, but there were proportionately more old people. Declining fertility and improved longevity of the population are thus reflected in the 1991 pyramid. For more information about this article or others presented in these column, write to the General Statistical Branch (1), Census and Statistics Department, Wan Chai Tower, 12 Harbour Road, Hongkong or call 582-4731.