Crooks (1988) reviewed over two hundred studies of the impact of classroom evaluation practices on students and concluded that the power of assessments to guide learning was not being realised because the summative function of assessment - providing grades and other measures of how much had been learned - is dominant. The results of many studies are given in terms of a “standardised effect size” [“effect size” for short: this following Cohen (1988) is the difference in performance between two groups (e.g. those given and those not given feedback) divided by a measure of the spread of scores in the population (the standard deviation)]. While the standardised effect size has undoubted advantages over reporting the level of statistical significance attained in experimental comparisons (Harlow, Mulaik and Steiger, 1997), it nevertheless suffers from limitations as a metric with which to compare findings from different experimental studies.