WHAT ARE SUBSCALE SCORES?
This is the first level at which item scores can be summed. If the items on the total scale are grouped into clusters related to topics such as instructional methods, evaluation methods, and course content, the item scores can be summed to produce subscale scores. These scores should only be used for decision making if adequate validity and reliability evidence support that internal scale structure. Since each subscale contains a different number of items, the score range will also be different. This range must be reported to interpret the results.
The summary of subscale score results is derived from the item score results. The statistics are the same. We are just aggregating or summarizing the item-level data (0–3) into subscale item clusters. For example, here are results for three selected subscales:
Instructional Methods (IM)—13 items
Subscale score range = 0–39; Midpoint = 19.5
Mean/Median = 34.79/37.00, where N = 97
Evaluation Methods (EM)—5 items
Subscale score range = 0–15; Midpoint = 7.5
Mean/Median = 13.40/15.00, where N = 97
Course Content (CC)—8 items
Subscale score range = 0–24; Midpoint = 12
Mean/Median = 21.97/24.00, where N = 97
COMPUTATION OF SUBSCALE SCORES: The interpretation of subscale results is analogous to the item results; only the numbers are BIGGER. For example, instead of a 0–3 range and a midpoint of 1.5 for an item, each subscale has a range and midpoint based on its respective number of items. So, for the Instructional Methods (IM) subscale with 13 items, a "0" (SD) response to every item produces a sum of 0 for the subscale, and a "3" (SA) response to all 13 items yields a sum of 39.
INTERPRETATION OF SUBSCALE MEANS AND MEDIANS: The zero-base for all score interpretations is easy to remember: the worst, most unfavorable rating on any item, subscale, or total scale is "0." What changes is the upper score limit for the most favorable rating on each subscale because the number of items change. Again, for the IM subscale, the mean and median can be referenced to the upper limit of 39 and also the midpoint of 19.5 to locate the position on the continuum, as indicated below:
Extremely Mean Extremely
Unfavorable Neutral 34.79 Favorable
0___________________19.5____________________39
Mdn
37
The mean/median ratings on the IM subscale are very favorable.
The subscale results can pinpoint areas of strength and weakness. They may be used by your department chair or the promotion review committee to identify your teaching strengths across different courses. Subscale scores cannot direct you toward particular aspects of teaching that can be improved or changed. The item and anchor results described previously are intended to provide that detailed level of direction.
Finally, the next blog will examine total scores on the scale. What additional info do they provide beyond what we already know?
COPYRIGHT © 2010 Ronald A. Berk, LLC
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