The percentiles and scores for the English language reading and mathematics tests in the “Progress Monitoring Scoring Guidelines” were calculated nationally-representative stratified norm sample, with 500 students drawn from each of four regions (West, Midwest, Northeast, Southeast) for a total sample of 2000 students per measure/per grade. These norms were developed in 2013-2014 and will be updated in 2020 (and on a five-year rotation thereafter).
Norms tend not to change too drastically over time. For instance, if you examine the Hasbrouck and Tindal ORF norms (Hasbrouck and Tindal ORF Norms), you will see that ORF norms have remained fairly constant over the past 25 years. However, it is generally accepted that student demographics shift over time, as do classroom teaching practices. Thus, most test developers update their norms every 5-10 years. Here at the University of Oregon, we opted to follow industry best practice in adopting a five-year norming cycle for the easyCBM assessments, beginning in 2020 and continuing every five years thereafter.
The percentiles for the Spanish language reading tests were calculated from a much smaller sample of students, K-2, who took the Spanish-language reading tests in School Year 2012-2013. The Spanish norms should be used with caution, as the norm group had only a few hundred students, rather than several thousand. This will also apply to the newly added vocabulary measures for grades 2-8.