Questions on the data
How accurate is this data? We know it will be totalled from actual results from SATs and in this sense we can assume a degree of accuracy in the numbers.
How reliable is the data? We cannot say with any real certainty. If we had the 95 per cent confidence interval for each annual mean we could perform statistical tests to evaluate the significance of the differences from year to year. We might assume it is reliable, but unless we know all the schools treated their pupils in the same way, for example, no 'practising for the test' took place, we cannot say for certain it is 100% reliable.
How valid is the data? Validity as we have seen is a property of the data so we need to define what it is we want to claim as valid.
Is there a general trend in improvement in results? Yes, there appears to be at age 11 if we assume that the data is reliable. However, to be certain we would need to perform further statistical tests.
Are KS2 pupils more able than KS4 pupils? Ignoring the question of whether these tests indicate anything about ability, the answer is no, because comparing pupils of different ages is like comparing apples and pears.
Did KS4 pupils have a harder paper than KS2 pupils? No, this is a completely subjective statement and cannot be validated by simply looking at this data.
Is it valid to say KS4 teachers are less effective than KS2 teachers? No, because the children are of different ages and the tests were different. We are not comparing like with like.