What error did the special education teacher make regarding Zac's percentile ranks?

The assumption that all percentiles represent equal intervals is incorrect due to the nature of percentile ranks themselves. Percentile ranks are a statistical measure indicating the relative standing of a score compared to a distribution—however, they do not reflect equal spacing between the ranks. For instance, a score in the 90th percentile indicates a higher standing compared to 70th percentile, but the difference in actual scores between these percentiles may not be the same as the difference between the 60th and 70th percentiles.

Understanding that percentiles do not represent equal intervals is crucial for interpreting data accurately. For example, if a special education teacher mistakenly treats percentiles as if they are evenly spaced, this can lead to incorrect conclusions regarding a student's performance and potential needs. This misunderstanding could cause the teacher to overlook essential insights into the student's progress or requisite support strategies.

Recognizing the concept that percentiles reflect a ranking rather than a graded scale emphasizes the importance of careful interpretation in the context of special education. This knowledge is pivotal to providing relevant and appropriate educational interventions based on the student's needs.