in particular where the range of outcomes is restricted eg studies on specific subpopulations such as students with special educational needs the effect size is inflated because the divisor in the calculation is smaller black and wiliam a 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 nevertheless as a general guide at least on standardised measures of educational achievement effect sizes of around which are typical in studies of feedback indicate an increase of at least in the rate of learning