It doesn't add up

With regard to your report "Does this new exam index work?" (Education, 14 January), how can we know that the factor in the calculations is not a square root, or a cube root, or something involving a bit of calculus or stochastic modelling. On second thoughts, forget the last one; Standard Life do that.

The fact is that we have no evidence at all that results have a perfect inverse correlation with free school meal numbers, which is what is used here.

Any relationship would be a highly complex one, demanding calculations on a Cray supercomputer, since every school, community, teacher and pupil is unique.

A formula that worked could not produce a figure for S5 pupils that is greater than 100 per cent gaining three or more Highers. As it stands, Dumfries High would need 536 per cent of its S5 pupils gaining three or more Highers to match the top school, Hillpark, a result which is clearly absurd, as is this piece of research.

(DR) WW FLOOD

Rowanbank Road

Dumfries