Given the small size of the effects for each individual student, even a slight bit of selection bias could dramatically alter the estimated benefits of an individual teacher. (educationnext.org)
The larger - than - expected size of the effects of this modest tax increase may be because the tax change occurred at the same time as the Great Recession — a time when unemployment was high and personal incomes lower, according to the study. (sciencedaily.com)
But while preliminary findings from the first 32 cases involved in the study confirm causality, the researchers said, the true size of the effect will become clear only after full analysis of all 200 cases and 400 controls. (businessinsider.com)