This average has not changed from when the first studies were published in 1989 and this provides some confidence in the robustness of this average. However, as stated in Visible Learning, care is needed in using this 0.4. If the tests are measuring a narrow concept then the effects can be higher than if they are measuring a broad concept. For example, the effects can be higher in junior compared to senior classes (often because of the narrow to broader notion). Any effect is only as robust as the measures they are based on. The time over which any intervention is conducted can matter (we find that calculations over less than 10-12 weeks can be unstable, the time is too short to effect change, and there is a danger of doing too much assessment relative to teaching). These are critical moderators of the overall effect-sizes and any use of .4 hinge point must take these into account.
Independent estimates, external to the research that is included in Visible Learning (2009), indicates that the average growth per year in mathematics and reading is approximately .4. This statistic was derived from an analysis of data from NAEP (USA), NAPLAN (Australia), SaTS (UK), and e-asTTle (NZ). There is a clear sense of average change – but again it is average and as noted above it is always critical to look for moderators (it is more like .5 in primary and .3 in secondary schools for measures of literacy and numeracy) and for these estimates of narrow excellence it is the interpretation that matters.