The mean score provides information about the "typical score" for students’ perceptions of the quality of instruction for a specific course but does not provide a complete picture. The five-point Likert scale used for the SCP survey is ordinal,not continuous: it uses a scale that arbitrarily numbers an ordered series of labels ranging from “strongly disagree” to “strongly agree.” With an ordinal scale, the difference between a mean score of 3.9 and 4.2 is not overly meaningful.
More about continuous vs. ordinal scales
A continuous scale measures numerical data. We can measure numerical differences in dollar amount. If Amy has $5 and Ping has $4, we can say that Ping has precisely 1 dollar more than Amy.
An ordinal scale orders nominal data (e.g., categories) to make it possible to measure it in a numerical way. For example, if Amy received an overall mean SCP score of 5 while Ping received an overall mean score of 4, we can say that Nancy obtained a higher score than Ping. To say that Amy is a “more effective teacher” by 1 point would be over-interpreting the numbers applied to the categories.
In fact, unless Amy and Ping taught the same course to the same cohort of students, any comparison of their scores is meaningless. All we can say is that a set of students rated Amy at a score of 5 and a different set of students rated Ping at a score of 4.
Consider the following fictional scenario:
Amir receives an overall score of 3.2. A closer look at the distribution in scores on the histograms for each item shows there are 4 extreme outliers (students who selected 2 on the scale for every survey item) but the rest of the scores are clustered between 4 and 5.
In this case, the mean score does not reflect most students’ perception of this course.
Be very cautious in assigning significant weight to Amir’s low average. This score is the result of only a handful of students’ experiences in this course. It would be advisable to examine Amir’s scores in other courses to get a clearer picture of student perceptions of his teaching performance.