Sports predictions

How a forecast is made

First the attacking and defensive strength of each side is worked out from matches played, adjusted for the opposition faced: goals against a bottom club and against a leader are not worth the same. Expected goals follow from those figures, and the outcome probabilities from them.

What data goes in

Match results from the past two years, weighted so that a recent game counts for more than one from last season. League position, head-to-head history and, in football, the average bookmakers' line. Injuries and suspensions are gathered separately and appear in the write-up.

What the probability means

Sixty per cent means that, were the match played many times over, the outcome would come up in roughly six cases out of ten. In four it would not — and that is not an error in the calculation but the nature of an estimate.

Limits of a forecast

The calculation knows nothing that is not in the data: a coach dismissed on the eve, the weather, a side rested before a European tie. For clubs with two or three matches behind them there is not enough to go on at all, and no forecast is made.