It will come as no surprise to horror fans that there is a formula to the genre - the success of the Scary Movie spoofs is founded on this.

Screenwriters may also be fairly blasé about the idea of the formulaic. Syd Field, in his writers' handbook Screenplay, broke scripts of all genres down to three acts, and said act two should run from page 30 to page 90, with a concluding act one "plot point" between pages 20 and 25.

But no one has so far gone to the trouble of breaking down horror's constituents into elements such as es (escalating music) and t (the sense of being trapped), and then welding them into this mathematical proposition.

As far as this new formula is concerned, it isn't clear what units will be used to measure each concept. Just as physicists use ohms for resistance and amps for current, perhaps horrorologists will now use yecchs for gore, helps for the sense of being trapped, aaaaaarghs for shock, herrmans for escalating music (after the Psycho composer Bernard Herrman).

There's one other concept I would add to this theory: e for evil. Almost all the great horror films succeed in evoking creeping, slithering wickedness at some level. Nothing beats Roman Polanski's satanic chiller Rosemary's Baby for that - with the possible exception of Abel Ferrara's vampire masterpiece The Addiction.

I feel that, mathematically speaking, the relationship between explicitness and a power to disturb is an inverse one. Less is more - but not always. The greatest horror film is probably The Exorcist, and that achieves much of its effects through explicitness.

As for the researcher who came up with this - how many hours did she spend working out her formula? Hours she could have spent enjoying the films. That is the scariest integer of all.

· Peter Bradshaw is the Guardian's film critic