[rkts]

Body size varies more within dog breeds than between them

I have never understood what this means. "Within-group" variation can be quantified as standard deviation (among other metrics). How do you measure "between-group" variation? Sample the group means and find the standard deviation? But that's obviously meaningless.

[thras]

Yes, it is fairly pointless. However, they do mean something exact. You quantify the "within-group" variation not with standard deviations, but by measuring all individuals in a group and finding the endpoints.

So you find the tallest white person in the world and the shortest, and note that they differ in height by a number of feet. Then you measure the difference between the white height average and the Asian height average, and note that the difference is only a few centimeters. Wallah! Race doesn't matter. Most variation is within group.

Like I said, I chuckle a bit.