Published: Dec. 14, 2020 By

Link to Resource: Comparison of 2019 Cohort and Baseline Student Growth Percentiles

Authors: Benjamin Shear

Suggested Citation: Shear, B.R. (2020). Comparison of 2019 Cohort and Baseline Student Growth Percentiles. Boulder, CO: The Center for Assessment, Design, Research and Evaluation (CADRE). https:// 


Each year, as required by state and federal statute, the Colorado Department of Education (CDE) produces accountability ratings for each school. At the Elementary and Middle school levels, these accountability ratings are based on two aspects of student performance on the state Colorado Measures of Academic Success (CMAS) tests - status and growth. Status is measured by the average scale scores students receive. Growth is measured by the quantile-regression based student growth percentile (SGP) model (Betebenner, 2009). Both metrics are also disaggregated by student subgroups. Although state statute requires measuring growth using the SGP model, there are multiple ways the model can be estimated. This report focuses on one particular technical choice CDE must make when deciding which version of the SGP model to use in measuring student growth, namely whether these measures should be based on so-called cohort or baseline-referenced SGPs. The purpose of this report is to: 

  • Compare and contrast the interpretation of baseline-referenced versus cohort-referenced SGPs.
  • Evaluate empirically how much inferences about student or school-level growth results might differ if baseline-referenced SGPs were used in 2019 instead of cohort-referenced SGPs.
  • Discuss issues to consider when comparing baseline and cohort-referenced SGPs. 

The analyses in this report were planned and carried out using data collected prior to the disruptions caused by COVID-19. This report is intended to provide background context and results relevant to the use of SGP data under standard test administration and reporting conditions and does not address concerns specific to the educational disruptions and challenges that have occurred during the pandemic.