shri - 2:43 am on Mar 1, 2011 (gmt 0)
Not necessarily a referral - even separate domains to process orders for the same companies would cause huge potential problems in that data.
If I remember DirectHit used to cookie users when they clicked on a result. If you came back and clicked on another result, some process would then determine that the initial site clicked on was low quality (simplifying this.. I'm sure they factored in time and a few other factors).
Here's a link to consider.