The graph (left) shows five countries where this effect varies greatly. It compares for each country one of the most underprivileged schools with one of the most privileged, in terms of mathematics scores of 15-year-olds in the Pisa test. The steepness of the line between them is an indicator of how much difference student intake makes to results.
In Germany, for example, the line is three times as steep as in Sweden, indicating that two schools with a given difference in the average social background of their students have a performance gap three times as great in Germany as in Sweden.
However, Sweden has fewer schools having to cope with extremely disadvantaged students, as shown by the fact that its line does not go as far to the left.
Spain, however, has a similar range of school intakes to Germany, but the effects of school intake on performance is not much steeper in Spain than in Sweden.
Incomprehensive school systems, the effect is relatively steep in countries such as Australia and the United States, which in Pisa 2000 were similar to the UK in this respect.
In contrast, Finland and Iceland show no association between social intake and school performance: these countries show that at least somewhere in the world, something close to equal educational opportunity does exist.