When looking at separate science outcomes, the SPI figure is calculated by comparison with all schools in that dataset. This means a non-selecting comprehensive school (where often only the more able will sit separate science) is compared against selective schools where all pupils sit separate science.
This has a number of affects:
It affects combined science outcomes as many higher ability pupils will not be included in the data as they sat Separate science.
It affects the separate science outcomes as pupils are compared against a selective intake with a very different educational experience.
It would be useful if we could compare data for our own school(s) with that of similar schools. This would provide a much more realistic picture of what our progress looks like.