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Percentile rank

Percentile ranks of greater than 25 or less than 75 are, in all but the largest schools, rarely significant (in terms of statistical confidence limits). A change, for example a rank of 40 to 60, represents a relatively small change in value added and is not be regarded as indicating an issue. A trend, though, from 20 to 40 to 60 to 80 over four years would be highly significant.

Percentile rank and contextual value added scores

Contextual value added scores (for instance in the snake plots) in RAISEonline have had a significance test applied to them using a 95% confidence interval. This results in a range of scores that may represent the true result for the school and thus a range of percentile ranks. For some small schools the range of percentile ranks reflecting the confidence interval could be large, such as in the example below.

A graph entitled 'CVA Mathematics' is shown. It plots Contextual Value Added Score on the y-axis and percentage on the x-axis from 100 to 0. A line on the graph shows percentile ranking.

An increase in percentile rank of the scores in the lowest 5% or the highest 5% of scores would indicate a much larger increase in progress than a similar change in percentile rank for, say, a school at the 50% marker. This is because of the unequal intervals that separate the percentile ranks. For instance changes in progress are higher to the far left and far right than in the middle of the snake plot above.

It is inappropriate to compare individual (or banded) percentile rank contextual value added scores. Use only changes or differences in significance.