Just to clear up a couple of things:
As said, ES (Extreme Spread) is the difference between the highest and lowest velocity in a particular string of fire whether that is a 3,5,10 or 100 shots.
Unfortunately this data is reliable for this string only and can not be interpolated into the entire population of shot (the rest of the shots you take for the life of the barrel).
SD (Standard Deviation) is much more useful. Assuming a random sample, then roughly 68% of the total population will fall within +/- 1 SD. 95% will fall within +/- 2 SD's. 99.7% will fall within +/_ 3 SD's.
With an ES of 50, on that day there was a difference of 50 fps in the string you shot.
Assume Joe 0121's SD of 11 fps.
11 fps * 3 = 33 fps 99.7 % of the shots from this load and rifle can be expected to fall within 33 fps of the average muzzle velocity of 2932 fps. This would give a max mv of 2965 fps and a min mv of 2899 fps, or an actual extreme spread of 66 fps for the total population.
As you can see, using this real world data as an example the actual extreme spread of the population is very different from the ES of the sample. ( 66 vs 27).
Standard Deviation is much more useful than Extreme Spread.
When you look at the statistics of shooting, don't forget to look at your target. It is not uncommon to have very nice group started, say 3, 4 or 5 shots that group within 1/2 moa. You are shooting over a chronograph and the mv is consistent with very little dispersion, say extreme spread for these is 15 fps. Then you have a flyer! Did you pull it? Check the chrono and the mv is 60 fps more or less than the others. For a string like this, ES would be high at ~60 fps, but SD would be low because there was a group with low dispersion followed by a single flyer. (If the mv of the flyer had been consistent with the others I would have assumed that I pulled it.)
One other point to consider. Say you shoot a group that is less than 1/2 moa, but your chronograph shows a high ES or SD. Do you trust what you see on the target or do you trust the chrono?
FWIW, just a few things to consider when looking at the statistics from your chrono.