VAMs v. Student Growth Percentiles (SGPs) – Part II

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A few weeks ago, a reader posted the following question: “What is the difference [between] VAM and Student Growth Percentiles (SGP) and do SGPs have any usefulness[?]”

In response, I invited a scholar and colleague who knows a lot about the SGP. This is the first of two posts to help others understand the distinctions and similarities. Thanks to our Guest Blogger – Sarah Polasky – for writing the following:

“First, I direct the readers to the VAMboozled! glossary and the information provided that contrasts VAMs and Student Growth Models, if they haven’t visited that section of the site yet. Second, I hope to build upon this by highlighting key terms and methodological differences between traditional VAMs and SGPs.

A Value-Added Model (VAM) is a multivariate (multiple variable) student growth model that attempts to account or statistically control for all potential student, teacher, school, district, and external influences on outcome measures (i.e., growth in student achievement over time). The most well-known example of this model is the SAS Education Value-Added Assessment System (EVAAS)[1]. The primary goal of this model is to estimate teachers’ causal effects on student performance over time. Put differently, the purpose of this model is to measure groups of students’ academic gains over time and then attribute those gains (or losses) back to teachers as key indicators of the teachers’ effectiveness.

In contrast, the Student Growth Percentiles (SGP)[2] model uses students’ level(s) of past performance to determine students’ normative growth (i.e., as compared to his/her peers). As explained by Castellano & Ho[3], “SGPs describe the relative location of a student’s current score compared to the current scores of students with similar score histories” (p. 89). Students are compared to themselves (i.e., students serve as their own controls) over time; therefore, the need to control for other variables (e.g., student demographics) is less necessary. The SGP model was developed as a “better” alternative to existing models, with the goal of providing clearer, more accessible, and more understandable results to both internal and external education stakeholders and consumers. The primary goal of this model is to provide growth indicators for individual students, groups of students, schools, and districts.

The utility of the SGP model lies in reviewing, particularly by subject area, growth histories for individual students and aggregate measures for groups of students (e.g., English language learners) to track progress over time and examine group differences, respectively. The model’s developer admits that, on its own, SGPs should not be used to make causal interpretations, such as attributing high growth in one classroom to the teacher as the sole source of growth[4]. However, when paired with additional indicators, supporting concurrent-related evidence of validity (link to glossary), such inferences may be more appropriate.”

 


[1] Sanders, W.L. & Horn, S.P. (1994). The Tennessee value-added assessment system (TVAAS): Mixed-model methodology in educational assessment. Journal of Personnel Evaluation in Education, 8(3): 299-311.

[2] Betebenner, D.W. (2013). Package ‘SGP’. Retrieved from http://cran.r-project.org/web/packages/SGP/SGP.pdf.

[3] Castellano, K.E. & Ho, A.D. (2013). A Practitioner’s Guide to Growth Models. Council of Chief State School Officers.

[4] Betebenner, D. W. (2009). Norm- and criterion-referenced student growth. Educational Measurement: Issues and Practice, 28(4), 42-51. doi:10.1111/j.1745-3992.2009.00161.x

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