Comparing Changes in the Percentage of Students at Benchmark in Two Schools

Another example described Principal Brown’s comparison of scores of her fifth graders on the state proficiency test to scores of the fifth graders in a nearby school. After discovering that her students scored significantly lower than others she and her staff worked diligently to change the situation. Three years later 70 percent of the fifth graders at her school scored at the proficient level, compared to only 50 percent at the earlier testing. At the same time the scores also rose at the nearby comparison school – from 65 percent to 75 percent proficient. Principal Brown and her staff knew that their students were doing better, but was the change enough greater than the change in the nearby school to be considered educationally or statistically significant?

Principal Brown used the EIC to answer her questions, and the results are shown below. Note that data are entered first for the two cohorts for the user’s group and then the data are entered for the comparison group. Data for the more recent year are entered first. Principal Brown and her staff would, no doubt, be gratified by the results. The positive effect size of .20 shows that fifth graders in her school had improved one-fifth (20%) of a standard deviation more than those in the comparison school. This is equivalent to a change, for the average student, of almost eight percentile ranks and would occur by chance only 4 percent of the time.

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