Confidence in results

Attainment and progress measures are based on a given set of pupils' results for a particular test paper on a particular day. The same pupils may have achieved different results on the day or the school would almost certainly have shown slightly different results with a different set of pupils, even with the same levels of prior attainment. Yet the school could be equally effective. This causes us a degree of uncertainty that we should take account of.

Confidence intervals

The uncertainty of a contextual value added score as a measure of school effectiveness can be presented as a confidence interval. This is a range of scores within which we can be statistically confident that the 'true' school effectiveness will lie, so a school's true result will be somewhere on the line but we do not know where.

Confidence intervals for individuals

Some analysis will produce a confidence interval for an individual result. Here we can see an example from MidYIS. The scores have been standardised, with a mean of 100 and a standard deviation of 15. Two thirds of the pupils will score between 85 and 115. Pupils scoring above 130 are in the top 2% of the population. The bars show the confidence limits. For example a pupil with a score of 100 might if re-tested score between 90 and 110.

Reproduced with permission from The CEM Centre, http://www.midyisproject.org

See also: Group size and statistical significance