DOCTOR Downing (The Expert, Evening Press, February 5) understates the incidence of the full autistic spectrum, that is the range of autistic disorders from the very mild to the very severe.

The National Autistic Society works with an incidence of one in 110 rather than his more hopeful one in 200.

The NAS figure may be the result of better research methods, different statistical models, more accurate diagnosis, even appropriate diagnosis. Perhaps it is due to a widening of the definition of autism; possibly it is the use of the most generous statistic to emphasise the problems of autism. Whichever it is, it shows that this disorder is a much more common condition than previously thought and one that needs appropriate responses.

It may well turn out, indeed I hope it does, that MMR has no autism effect. But the incidence of autism has risen and one must ask why?

It is time we tried to find out.

Anthony Still,

Barbers Drive,

Copmanthorpe, York.

Updated: 11:31 Thursday, February 15, 2001