Principal
Margaret White was interested in differences in scores of students in two third
grade classrooms in her school. The students had been randomly assigned to
teachers at the beginning of the school year, and they had very similar skills
at that point. Each classroom had 25 students. Yet at the end of the school
year the scores in Classroom A, where the percentile rank of the average student
was 66, seemed markedly lower than the scores in Classroom B, where the
percentile rank of the average student was 78. Principal White wondered if this
difference was large enough to be considered educationally significant or if it
could have just occurred by chance. To answer that question data were entered
into the EIC. The results
from the EIC confirm Principal White’s concerns. The effect size of -.36 would
be seen as indicating that the gap between classroom A and classroom B is
educationally significant. The probability level of .20 is above the .05
cut-off that is often used, but that no doubt reflects the relatively small
number of students in each group. Principal White would probably want to
consider corrective actions.[1]
| Data for Your Group | |
| a) Percentile rank of average score | 66 |
| b) Number of students tested | 25 |
| Data for the Comparison Group | |
| a) Percentile of average score | 78 |
| b) Number of students tested | 25 |
| Results | |
| Effect Size | -0.36 |
| Improvement Index | -14.0 |
| Probability this effect would occur by chance | 0.20 |
To enter your own data Click Here
[1] Careful readers will note that the improvement index is not
equivalent to difference of the two percentiles used as input data. That occurs
because of the differences between percentile scores and the NCE scores used in
calculations. One could argue that the Improvement Index calculated with NCE
scores provides a more accurate estimate of the effects than simple comparisons
of the percentiles. (See NIFDI Technical Report 2016-2 for more details on calculations. To see the Report Click Here)