In particular, where the range of outcomes is restricted (e.g. studies on specific sub-populations such as students with special educational needs), the effect size is inflated because the divisor in the calculation is smaller (Black and Wiliam, 1998a). Second, measures of educational outcomes differ greatly in their sensitivity to the effects of education and whether the measure relates directly to what students have been learning or is more remote, as with many national tests and examinations (Wiliam, 2008). Nevertheless, as a general guide, at least on standardised measures of educational achievement, effect sizes of around 0.4, which are typical in studies of feedback, indicate an increase of at least 50% in the rate of learning.