HOW much do you know about the population of Hongkong? Possibly, you have drawn an impression through daily observations of the people around you. To get a real understanding of our population, which comprises several million people, we actually need to refer to population statistics. Among the various sources of such statistics, population censuses are obviously the best known one. The last population census was conducted in March 1991. There are many findings in the 1991 Population Census and these can be compared to those of the 1981 Population Census. One fact revealed is that the Hongkong population is ageing. For a person, he definitely gets older between two time points which are 10 years apart. For the population as a whole, this is not necessarily so. Births, deaths and migration (both immigration and emigration) that go on all the time have different effects on the age distribution of a population. For a population, the number and proportion of people vary from one age bracket to another. The profile of this variation is called the ''age distribution'' of the population. In describing a population as ageing, demographers usually make references to its ''median age'', which is calculated on the basis of the population's age distribution. It is the age which divides the population into two halves when all individuals in the population are ranked according to age. That is to say, half of the population is above its median age and the other half below it. When the median age of a population increases over time, it may be said to be ''ageing''. When the median age decreases, the population may be said to be growing younger. The median age of the Hongkong population increased from 26 years in 1981 to 32 years in 1991, as revealed by the population censuses taken at those two time points. Demographers generally agree that a population with the median age under 20 is young; and one with the median age of 20-30 is of intermediate age. By this standard, our population has reached an old age. We are still younger than Japan (around 37 in 1989), about the same as the US (around 33 in 1989), but much older than China (around 25 in 1990). It is important to study the age distribution of a population because there are different implications for society and government planning. Having considered the median age, which is one item of statistics that describes the age profile, we can form anidea of what community facilities are required. Thus, when we know that a population is getting younger, more schools and related amenities are needed. On the contrary, for an ageing population, more medical services, elderly care and so forth should be provided. Currently, Hongkong fits into the latter category. Naturally, we need much more detailed analysis to understand the population situation thoroughly. In some of the articles to appear later in this series, other relevant statistical measures will be introduced. For more information about this article or others presented in columns, you are welcome to write to the General Statistics Branch (1) of the Census and Statistics Department at Wan Chai Tower, 12 Harbour Road, Hongkong, or call 5824731.