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?
- Nothing. You report it as is, don’t attempt any statistical testing, and hope it doesn’t happen again.
- 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:
- 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).
- 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.