Why do some scores have a large percentile range?

The percentile is determined by two factors: The Rasch Unit (RIT) score itself (and where that score falls in the distribution of other scores in the grade level as they existed in the norming study) and the measurement error associated with the score. For example, two grade 3 math scores in the spring have a measurement error of 3.0. The two scores are 201 (52 percentile rank) and 223 (97 percentile rank). The RIT ranges (and the percentile ranges) for these two scores are:

• 201   198-204 (41-61) 

• 223   220-226 (95-98)

So, given the same level of error, scores toward the middle of the distribution will have a wider percentile range. When the student's achievement level is measured with less precision (i.e. the measurement error >4+), the effects are even more pronounced in the middle of the distribution.