WE ARE living in an age where statistics are used more than ever before to guide education policy and evaluate performance. Information technology has given us the capacity to handle huge amounts of complex data. However, we can only analyse statistics if they have previously been collected by somebody. We are therefore in danger of ignoring crucial factors simply because they don't happen to have been counted.
There is no more glaring example of this than pupil mobility in schools. Some schools have stable populations, whereas others have high rates of pupil turnover.
In some urban areas, and particularly in London, there is extensive movement of children into and out of schools throughout the year. In one inner London authority, more than a third of children taking key stage 2 tests last year were not on the roll of their current schools at key stage 1. In another, a medium-sized secondary school had 268 children joining or leaving it during 1996-97, in addition to the normal intake at age 11 and leavers at 16. But there are no national statistics on the subject, so it tends to be ignored.
This is unfortunate for the schools concerned. It is clearly absurd to judge the quality of education in a school on the basis of test or exam results without knowing about its pupils' mobility. An inner-city school with poor achievement in literacy at key stage 2 may be a failing school. It may, equally, be a good school which regularly loses its high- achievers through outward movement to the suburbs, while replacing them with children who have language or learning difficulties. Without the statistics, who knows?
Pupil mobility is nothing new. In 1967, Plowden reported: "We saw admission registers whose pages of new names with so many rapid crossings out told their own story of a migratory population. In one school, 111 out of 150 pupils were recent newcomers."
The well-known research by Peter Mortimore and others in London junior schools 10 years ago painted a picture similar to the present.
Nor is high mobility just a London problem. Manchester cited it as a key issue for its education service in its evidence to MPs on performance in city schools in 1995, while a parliamentary question by Birmingham MP Lynn Jones earlier this year about "the special needs of primary schools with a high turnover of pupils" indicated that it was an issue there too.
However, the Government's White Paper Excellence in Schools made no mention of pupil mobility in its 84 pages, except where it appeared to refer to movement between stages of education. Yet many of the policies currently being enshrined in legislation pose particular problems for the high-mobility school.
Agreeing a home-school contract with every parent makes very different demands on a mobile school compared to a stable one. Setting targets for aggregate pupil achievement two or three years hence is a very different exercise in a school where all the likely candidates are known, compared to one where the pupil population is always changing.
Devising a literacy strategy is very different in a school where children move steadily up from Year 1 to Year 6, compared to one where beginners in English and English-speakers with low achievement levels join the school at unpredictable times in both infant and junior phases.
Mobility doesn't make national policies wrong. It does demand an understanding of the issues by those judging their implementation - and by those funding it.
Recently, two articles (TES, September 11) dealt with matters where pupil mobility has enormous relevance but didn't mention it. Roy Hattersley's piece on examination league tables could usefully have added it to his analysis of why these tables compare the incomparable.
Tony Travers' briefing on the formula for distributing education funding to local authorities (entitled "Statistically extremely significant for schools") might have considered the cost implications of high pupil turnover - that is to say, the cost of staff time, support services, books and equipment.
Much, though by no means all, of the movement in high-mobility schools involves children from ethnic-minority backgrounds.
While it is true, as Travers says, that some ethnic-minority groups are performing above the national average in exams, it is also true that the ethnic factor has helped to channel money to many local authorities and schools coping with high pupil turnover.
In so doing, it has contributed both to the success of many recent immigrants and to schools' capacity to meet the needs of other pupils, black and white, mobile and immobile, in a volatile and pressurised situation.
Now it appears that many such schools could lose out under new funding arrangements if additional educational needs are assessed on a different basis. The absence of national statistics on pupil mobility make it difficult for central government to build it in explicitly as a factor, even if its significance were acknowledged. If it isn't counted, it doesn't count.
Dr Janet Dobson is a senior research fellow at University College, London's migration research unit. She is planning a research project on pupil mobility and would be interested to receive any information or views on the subject from 'TES' readers.
* Ted Wragg, back page