Why should grade equivalent units not be used in mathematical calculations, such as determining the mean?
The grade equivalent scale is like a ruler that has inches of different lengths. It is not an equal-interval scale. The grade equivalent units do not represent equal amounts of ability at different points along the scale. A student who moves the same number of grade equivalents at one level on the scale, such as from 2.5 to 2.9, has not necessarily “grown” in ability the same amount as a student who moves the same number of grade equivalents at a different level on the scale, such as 8.5 to 8.9. The amount of growth in ability needed to move from 2.5 to 2.9 is much greater than that required to move from 8.5 to 8.9. Because grade equivalents are not equal-interval units, they should not be used in mathematical calculations, such as averaging. The Lexile and Quantile scales, in contrast, are equal-interval scales. Regardless of the point on the scale, the amount of growth in ability required to move between two points is the same. In other words, moving from 240L to 340L on the Lexile s
