I think scaling could be very useful to measure a specific skill that was learned during a course.
For example, if you were teaching negotiating or presentation skills, it could be used to measure a student going into the course with very little competency to a much higher competency.
Scaling in a short course setting like a college semester or corporate training program is good where under good conditions a student has the potential to measurably move two levels or more. In that way you will be able to measure students who did very well, normally, and poorly.
The problem with measuring general language ability with scaling is for most of our students and in the school setting, language learning is very incremental and the difference in general English ability from one semester to the next or even one year to the next is often unnoticeable on a scale.
I’m not saying students don’t progress. They do. But it is sometimes undetectable on such a scale.
I asked my students to make some business phone calls to America. I found their phone skills in this situation were rather poor. They had difficulty with the way their phone call was answered by a recording and the “tree” of choices, “Press 1 for information about your order. Press 2 for…” And then they needed to talk to a salesman and he often said things that sounded completely strange to my students, “You asked about XYZ? Okay, just a minute while I pull that up.”
This is something that we could have some lessons on and they could improve after some practice to being quite proficient in navigating the phone tree and then talking to a salesman. We could have a 5-point scale for this and measure how much the students improved. The normal gains would not be so incremental.
A scale would be a great way to measure if and how much a student progressed on a skill that they were very poor at but became or could have become proficient at from the beginning of the measurement to the end of the measurement, ie: during a short course.
I have taught some managers who were absolutely desperate to learn English in six months. They went from about IELTS Band 3 to Band 5 in that time. They had four hours of face-to-face training per week, did homework and spoke to me by phone every day.
A scale is useful in measuring that kind of progress, too.
However in the realm where most of my students are, hundreds of college students, the language skill gain is incremental year by year. And even the best measures may show a negative gain.