Comparing Changes in One Group with Changes in a Larger Group – Percentile Ranks of the Average Student

The teachers at HIllside School were proud of the change in their students' achievement over the past few years. For instance, in the most recent year the average student scored at the 60th percentile on the district's standard test of achievement. Three years earlier the average student was at the 40th percentile. One hundred students were tested each year. 

At the same time, scores in the district, which had always been higher than those at Hillside, were also rising. In the most recent year the average district student scored at the 78th percentile, while three years previously the average district student had scored at the 76th percentile. The HIllside teachers wanted to know the extent to which the change in achievement among their sixth graders was greater than that of sixth graders in the district.

To answer this question they entered data in the EIC and got the results shown below. The effect size of .44 is well over the .25 threshold and the probability level of .002 indicates that the results would be very unlikely to have occurred by chance. The Improvement Index of 17 indicates that the improvement of the average student in Hillside was 17 percentile ranks greater than the improvement of the average student in the district. In short, the results of the EIC indicate that the Hillside teachers could be justifiably proud of their students’ accomplishments.

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