Same question, two different scales

It happens.  Your carefully crafted  evaluation questions are administered to the survey population using a different scale pre- and post-activity.  Miscommunication, cut & paste fail, whatever the cause…what do you do with the data?

  1. Nothing.  You report it as is, don’t attempt any statistical testing, and hope it doesn’t happen again.
  2. Transform.  Call on your inner MacGyver and make these two scales compatible.

Tempting as option #1 may be, this blog wouldn’t be much use if we take that route.  So here are the simplest fixes:

  1. Proportional transformation: if you want to make a 5-point scale talk to a 7-point scale, you multiple each 5-point score by 7/5 (alternatively, you could reduce a 7-point scale to 5-point by multiplying each 7-point score by 5/7).
  2. Transform each score (e.g., all 5-point and 7-point scores) to a standard z-score using the following formula: z = (raw score – mean of raw scores)/standard deviation of raw scores.

In this case, simple may also be right (or right enough).  To see how these approaches compare to more complex transformations, check out this article.

 

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Filed under CME, data, Likert scale, Statistics, transformation

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