Good Math, Bad Math, ???? description

Over at Good Math, Bad Math, the author writes (in other articles) about how people don't "get it" with maths ...

Generally those articles are fun to read. But on many an occasion the author's posts themselves are corrected by some commenter or other. But this obviously is a lesser sin. The author does pick the right things to object to. And they might make those mistakes mainly because of the nature of good blogging : that which is done in the heat of the moment, without too much polishing etc.

Now with that preamble laid out...

In the article linked to here, the author says
The standard deviation, which is usually written σ is a root mean-square measure - which means that it's the mean (average) of the square root of the difference between the points and the mean squared. The sum of the squares is also a useful figure, called the variance; the variance is just the mean of the squares - that is σ²

The RMS expands into root (of the) mean (of the) square(s of the deviation from the mean)
not mean of the root, as described above. In fact, mean of the square root of the difference between the points and the mean squared is a cool measure too. Its something like the first moment of the absolute difference or the mean absolute deviation (when the positive root alone is considered).