The great majority of teachers assessed by this system do not differ from the average at conventional levels of statistical significance. (educationnext.org)
As a result, the annual impacts of the intervention must be as much as 20 to 30 percent of a standard deviation in order to be detected at conventional levels of statistical significance. (educationnext.org)
In four other surveys, the effect goes in the expected negative direction, even if it does not reach conventional levels of statistical significance. (blog.politics.ox.ac.uk)