Hi Alan,
The phrasing “within 2 SDs” refers to the standard deviation on both sides of the mean – i.e., 68% of values fall between 1 SD below the mean and 1 SD above it, as shown on the graph. The same applies to the other numbers: 2 SDs below and 2 above the mean is a total range of 4 standard deviations.
I can see how this phrasing is confusing, so we’ll look into updating the article to clarify – but I hope that clears things up for now!
]]>