Can you explain the information displayed on the AIMSweb Aggregate Norms table?

AIMSweb® Aggregate Norms:

The AIMSweb Aggregate Norms include multiple years of data from virtually all AIMSweb customers across all 50 states.  (Strategic and Progress monitoring data are excluded

from the AIMSweb Aggregate Norm.) The resulting normative table illustrates the performance by percentile rank of all students within the United States who took the specified

AIMSweb assessment, and for whom customers have subsequently recorded their scores manually or via electronic data capture tools within the AIMSweb® system.   In summary, the AIMSweb® Aggregate

Norms table is a summary of how students are performing nationally on various AIMSweb® measures.

Median:The median score (50th percentile) represents the midpoint between the highest and lowest scores achieved. 

Mean: The mean score is the average of all scores across all students' scores.

Standard Deviation:  The standard deviation is a statistic that expresses how tightly students' scores cluster around the mean.  The more that scores are spread apart within a data set, the larger the number that represents the standard deviation will be.  In contrast, the more tightly a data set clusters around the mean, the smaller the number that reflects the standard deviation will be. 

Statistically, sixty-eight percent of the data set falls within one standard deviation of the mean (For example, if the mean is 50 and the standard deviation is 10, 68% of students scores in that sample will fall between 50 +/- 10, or between 40 to 60).

The Rate of Improvement (ROI):  Rate of Improvement offers an estimated average weekly growth rate for students, based on a typical 36 instructional week school calendar, with approximately 18 weeks occurring

between fall to winter benchmark periods, and 18 between winter and spring benchmark periods.  On this report, the growth rate—or ROI—for students’ scores falling in a specific percentile rank is calculated by dividing the net growth (i.e., gain scores) from one or both benchmark periods by 18 or 36 weeks, respectively.