have planted plots for many years.. Always try to do a fair job with each variety. Pick a field that although may not be the best ground,will at least be the most even across the field.. Then spread fertilizer, spray etc, cross all rows. When the time comes for the weigh wagon the dealer or fertilizer wagon guy gets a page that say a b c d e and so on.. When the test is done and numbers tallied he then gets the variety numbers.. We don't plant a big plot,mostly so the ground stays the same.
  Few things you notice over the years,, early corn even in the middle of the field will have damage,animals or birds. So keep the days the same or very close.The one you really like may be gone in couple years so keep looking. does work  to weed out the non standers. And don't go by looks.. many times the best looking don't follow up as best yield.  And the same yield but five points wetter does nothing for us.
  But over the years the plots have helped.. especially when gauged against other plots in the area.