"When you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." This observation scores high on the list of silly remarks made by clever men, to which its author, Lord Kelvin, made several sterling contributions. The knowledge possessed by Plato and Shakespeare, Austen and Darwin was neither meagre nor unsatisfactory.

Bogus quantification attempts to compress complex problems and analyses into single observations. The annual report of the United Nations Human Development Programme is an entirely admirable endeavour. The agency focuses on three broad dimensions of development - health, education, and material standard of living - and reports on progress towards these goals among UN members.

Then the curse of Kelvin strikes. Health is measured easily enough. Take life expectancy at birth, subtract 25 years, then divide by 60. This score has a one-third weight in the final total. Educational development is measured as a combination of attainment - adult literacy - and opportunity - enrolment. Material standard of living is measured by gross domestic product per head at purchasing power parity. But differences in GDP are much larger than differences in life expectancy, so you prevent this measure from swamping the whole calculation by using the logarithm of GDP rather than the level. Finally, you give equal weight to each of the three components and come up with your ratings. Iceland, ironically, comes top, and Sierra Leone bottom.