On the forum "Accuratereloading.com", the thread "Effects of Tiny Variations" on velocity yielded some good thoughts. It would be worth reading.
It also has links to these articles which are worth reading, too.
and I wish I could find more details about this demonstration performed regularly in a master loading class conducted by a bullet manufacturer.
Originally Posted by Rat Motor
Load 10 cartridges over a scale and mark them number one with a felt tip pen. Use the same components and load ten cartridges straight from a good measure and mark them number two. Put all 20 into a bag and mix them up. Use two targets marked one and two. Dip into the bag and if you pull out a one fire it on target one. Number twos go on target number two. Shoot at one minute intervals and compare the two groups.
or compare the strings of velocities for uniformity.