Let us be glad for their own sakes that an equally severe test of brain capacity has not been required of Hong Kong government officials who pronounce every month on the latest unemployment figures.

They regularly seem to have difficulty in understanding that the unemployment rate is a function of two considerations.

The first is change in the labour force, which is the number of people either employed or looking for employment, and the second is the change in employment, the number of people who actually have jobs. You pull out your calculator and subtract the number of people employed from the number of people in the labour force to get the number of people unemployed. You then divide the number of people unemployed by the number of people in the labour force, multiply the result by 100 and, hey presto, the result is your unemployment rate, which last month was 8.5 per cent.

If you want to be fancy about this you then use a computer to apply a seasonal adjustment formula and you get the officially reported number of 8.3 per cent.

This, however, constitutes at least two thoughts at the same time and it is so much simpler to stick to one alone. Hence the common assumption in the halls of government that the labour force is absolutely static and all changes in the unemployment rate result only from jobs lost or jobs gained.

Hence also Financial Secretary Henry Tang Ying-yen's latest comments on unemployment, which focused entirely on increases in jobs and made no reference to changes in the labour force. Let us not speculate on what would happen if someone were to say to him: 'And, Mr Financial Secretary, you must also remember ...' The consequences could be grim.

The simple fact, as the first chart shows, is that the official (seasonally adjusted) unemployment rate dropped to 8.3 per cent last month from 8.6 per cent in August because there was a decline of 17,700 people in the labour force. The number of jobs also declined, however, but did so by only 6,000 and thus the gap narrowed and unemployment was still reduced.

Perhaps it is true that the labour force declined because many people decided to go back to school rather than look for jobs that could not be found. I think there may be another explanation, however. As the second chart shows, the number of immigrants we took in from the mainland last year was the lowest since 1994.

Why should it have gone down? Well, perhaps people across the border also decided that there was not much reason to go to Hong Kong if there were no jobs to be found here. In other words, our unemployment rate may be declining from natural causes of supply and demand pressures in the labour market.

I do know, however, that there is little evidence in the published figures for Mr Tang's assertion that 'many of the improvements have been in the travel service, retail and restaurant-related industries'.

Leave alone a curious definition of 'improvements', the fact is that employment in these fields has trended steadily down for the past two years and the unemployment rate in the restaurant and hotel sector alone stands at 14.5 per cent, slightly down perhaps from the height of the Sars scare but not much.

I also suspect that Mr Tang's emphasis on job creation in small and medium-sized enterprises (no numbers given of course) was made so that government can claim all the credit if unemployment now continues to go down.

But that would be three thoughts at the same time. Oh no!