Contextual value added snake plot
Press the PLAY button to plot all schools nationally onto the snake plot below.Each is indicated by its pink confidence interval and schools with small confidence intervals are obscured by the others. The blue line indicates each school's contextual value added score in percentile rank order.
In reality the percentile rank of a school can be misleading as the actual score might lie anywhere along the confidence interval. The middle 80% of schools have scores that are reasonably close to each other and so even a small difference could greatly affect the school's percentile rank. So when using a snake plot you need to base your judgements on whether the school's score is significantly different from the national average rather than on the ranking or left–right position on the graph.
Sig +
Here the school is significantly above the national average in terms of progress its pupils make to Key Stage 4.
Not sig
Here we can see that the confidence interval crosses the Key Stage 2 median line. We cannot be certain that the school's progress is above, below or even the same as the national average.
Sig –
The progress score here is very low, within the lowest 5% national of schools in the country. It is unlikely that we can ascribe this lack of progress to the pupils' background and more likely to the school.
See also:
Confidence intervals
Group size and statistical significance
The school contextual value added score
Percentile rank.