PIPS uses standardised scores with a mean of 50 and a standard
deviation of 10.
To convert them to a mean of 100 and a standard deviation of 15:
Multiply by 1.5 (this will adjust for the standard deviation)
Add 25 (this maps to the required mean)
...

We use a (weighted) average of maths and reading as a predictor
because this predictor correlates better with future attainment in
maths (and predicts more accurately) than maths alone does, and it
correlates better with future attainment in reading than ...