What is an "error"? or "Are all errors created equal?"

Background:

Research for my MA TEFL dissertation. I have 20 Japanese HS students who have been doing 10 minute free writing at the start of each class (2x/week) since April 2008. I plan to measure fluency (word count) and either accuracy (errors per 100 words) or complexity (vocab or grammar choice).

Is there literature on what defines an error? If anyone can point me towards a name, an article, or an opinion, I'd be very grateful.

Does anybody read these forums? The whole site is an excellent idea (Thanks Paul) but I wonder if people interact?

Cheers from Osaka, Japan,

Steven Herder