The recipe for "Thicket, Brush and Scatter" loads is about 100 yrs old; the shot is divided into 3 layers with cards. The technology has improved somewhat in that time.

One of the better discussions of spreader loads occurred here:

http://www.16ga.com/forum/viewtopic.php?t=5701&highlight=spreaders

As always, it's up to you to filter what you read.