Do the percentiles reported on MAP reports equate to specific standard scores, as one would normally find in a normal distribution of scores (Bell Curve)? E.g., does the 16th percentile equal an 85 Standard Score as one typically finds on many IQ and Achievement tests?
Yes, like the norms for many other standard assessments, NWEA’s norms are shaped like a bell curve, given the subject, the student’s enrolled grade, and the number of prior weeks of instruction received. In other words, for a student in a particular grade and with a particular number of weeks of instruction, her/his RIT score can be evaluated against a bell curve to determine how the student’s score compares to a nationally representative norm group.
The easiest way to know the percentile rank associated with a student’s MAP or MPG score is to read it directly off the report. However, those percentile ranks are computed in precisely the same way as with other standard tests, using the mean and standard deviation of the norming group. With IQ tests like the Weschler Adult Intelligence Scale (WAIS-IV) or the Stanford Binet (SB5), the mean and standard deviation are set to 100 and 15, respectively. With NWEA assessments, the distributions of performance are conditioned upon grade, subject, and instructional weeks, with the specific means and standard deviations provided by the latest norms documentation.
As you correctly point out, a student with a WAIS or SB score of 85 is performing one standard deviation below the mean, or at about the 16th percentile. A score of 85 on the WAIS would be associated with a standard score of -1 (that is, one standard deviation below the mean). Similar interpretations can be made with MAP scores. A student whose observed achievement was about one standard deviation below the mean, given grade, subject, and instructional weeks would receive a percentile rank of 16.