The best analysts shape raw data in a creative way that decision-makers can use to guide their actions. Data on exam results, for instance, conveys almost nothing about how much a school does for its pupils. However, by assigning a number to each of the 5-14 levels and looking at the gains made as pupils progress, the value added by a school can readily be seen.
The most instructive part, says Mr Mitchell, comes when you compare schools not just with national averages but with other schools across the country that are similar in terms of social and economic deprivation.
"It is also very useful to chart the performance of a school from year to year. Look at this," he says, pulling up a graph of value added in various subjects at one school over a six-year period, which shows an obvious dip in all subjects in 2000. "This sort of chart can draw attention to a problem needing attention or highlight an initiative that has been particularly successful."
A key point about all this data analysis, says Mr Mitchell, is that it demands no extra effort from school managers. "They already enter all sorts of data - attendance, exclusions, exam results - into the management information system. I just sit behind that and suck the data out. Then I combine it with data from the Scottish Executive about other schools."
The value added approach can be used to quantify and present what schools are doing for their pupils at any stage, particularly at the transitions from primary to secondary, 5-14 to Standard grade and Standard grade to Higher.
"Schools and teachers appreciate this approach because it reflects what they and their pupils are actually achieving. As one teacher said: 'I didn't make my targets this year but I did have very good value added.' The point is that value added takes account of where the kids start from, not just what level they reach."
On school visits, quality development officers arrive armed with the latest charts on value added in every subject at every stage and use them to inform discussions about achievements and targets.
"We can also look at other data such as attendance and exclusions, and again use the results to drive policy," says Mr Mitchell. "For example, we analysed good practice on exclusions and shared the results, and most schools have since seen a reduction in these.
"Whenever I can, I like to get from big numbers to individuals, to help teachers tease out extra bits of information to help their kids. But I'm always impressed by how much teachers already know about the pupils in their classes."