HOW DOES TOTAL SCORE COMPARE TO GLOBAL ITEM SCORES?
On many scales, global items are included at the end. These items ask students to provide a summary rating of the instructor and/or course. They take on a variety of formats, but the purpose is the same. For example, using anchors ranging from Excellent to Poor, the following items might be given:
What was the overall quality of your instructor’s teaching?
What was the overall value of this course?
OR
using Strongly Agree to Strongly Disagree,
This is the worst instructor on the planet.
This course sucks.
LIMITATIONS: The item response to each of these items would be interpreted the same as any other item on the scale. The problem is that these items are not diagnostic for teaching improvement. They provide an overall rating.
So what’s the problem? Individual item responses, either percentage responses to anchors or item means/medians, are usually unreliable. When those responses are used to suggest areas for improvement, they serve as a guide. No major career-shattering decisions are being made. If global item responses are used for summative decisions by your department chair or the promotion committee, there is a lot more at stake.
RECOMMENDATION: Subscale or total scale scores that summarize the quality of teaching or the value of the course are usually more reliable, based on a collection of items measuring those characteristics, than just a single item. It is recommended that those scores be used in lieu of global item scores whenever possible for any summative decisions about teaching performance.
My final blog in this bloated series will briefly describe criterion- and norm-referenced interpretations of the scores previously defined.
COPYRIGHT © 2010 Ronald A. Berk, LLC
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