Cross-posting here, I guess, but it’s totally relevant! 
I’ve written a pretty extensive explanation of all kinds of aspects of error and how to reduce it with the learning routine. It may be a bit much to read in one shot (16 pages, including numerous illustrations), but I think people will find it very informative and helpful. It’s a first draft of sorts, so suggestions are welcome!
Here it is:
http://www.wxsim.com/wxsim_accuracy.docx
Tom