Comparing Group Test Results
Since the Stanford 9 is unchanged from year-to-year and is administered in all California public schools, the test scores may be compared from year-to-year as well as between and among schools and/or districts.
The most defensible comparison is the Percent of Students Scoring At or Above the 50th NPR.
This is the percentage of students in the group purported to have demonstrated achievement at or above grade level on this particular test.
A number of comparisons are possible, each with its own set of cautions.
Same Year, Within School Comparisons
A reviewer may want to compare the performance of students at different grade levels within a school.
Similarities and differences in student performance in the same subject may be seen by comparing the percent of students scoring at or above the 50th percentile for each grade.
When making this comparison it is important to remember that the number of students in each group affects the confidence in the inferences that can be made.
The smaller the group the more cautious one should be in making comparisons.
It is also important to note that the national norm groups to which California's students' scores are compared were unique for each grade level.
Same School, Different Years Comparisons
There are two ways to compare two or more years of data for the same school.
One can look at a cohort comparison over time by following a group of students from grade-to-grade.
For example, if 48% of a school's third graders scored at or above the 50th NPR in 1999, 51% of the fourth graders scored at or above the 50th NPR in 2000, and 54% of the fifth graders scored at or above the 50th NPR in 2001, the school appears effective in improving the achievement of this group of students.
When making this comparison, it is important to understand that even if the number of students is the same from year-to-year, that the group's composition may be quite different if student mobility (transiency) is high.
In a cross-sectional comparison, last year's third-grade results are compared to this year's third-grade results.
Since the results for two-separate groups of students are being compared, differences that may exist between the two groups should be considered.
Scaled Score Comparisons (Cohort)
While scaled scores cannot be compared between different tests or subject areas, scaled scores are useful for comparing performance over time on the same test.
For example, if the second grade in a school had 52% of the students scoring at or above the 50th percentile and the 52% of the students also scored at or above the 50th percentile in third grade, a comparison of the average scaled scores may be used to determine if the students actually demonstrated growth during the year.
If a group maintains the same position relative to the norm group, the average scaled score will increase, because the average scaled scores for the norm groups increase from year-to-year.