I was the departmental statistician for Hertfordshire education authority from 1974 to 1999 and I provided school-place managers with estimates of long-term average demand by small area annually from about 1985. The original purpose was to ensure that school sites were not sold off that might be needed later. The same estimates can inform decisions as to whether an increase in demand for places is likely to be permanent or temporary and so best met with permanent or temporary capacity.
Estimates of average demand over a generation can be calculated and, provided the underlying assumptions are made plain, they can help in making sensible decisions. Examples of the use of long term averages can be found in the Hertfordshire School Organisation Plans published in 1999, 2000 and 2001, although not 2002.
So, the answer to Mr Whitfield's question is: "I can".
J R Walker 3 Larkens Close Puckeridge, Hertfordshire