When using a chronograph, you shoot a string of shots, (usually 10 rounds), and get an average velocity. The difference between high and low is the extreme spread. The difference between each shot and the average is the deviation for that shot. The standard (or average) deviation is the average of all of the deviations. The smaller the number, the better. I read somewhere, (would have to look it up to find out where) that the extreme spread of a load should be no more than 2% of the average velocity, or vertical stringing could become very noticable at the longer ranges.