The percentiles and scores for the easyCBM tests 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 are updated every five years, with the most recent update taking place in 2020.
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.