I know we recently discuss this issue (of whether merchant advertisers should allocate all credit for a sale to the "last click" or should apportion with other advertising & promotion efforts); here's an article on ClickZ on this very topic: http://newsletters.clickz.com/ct.htm...,58q8,a4gj,cer

The article refers to this report from Atlas: http://www.atlassolutions.com/upload...channeldmi.pdf

I think that the primary focus in the Atlas report is on "repetition," which is the notion that each exposure increases the consumer's likelihood of responding at some future time; thus, a consumer who has seen several display ads will be more likely to click on the company or product when it appears in search results.

This reminds me of my "internet advertising measurability epiphany" experience during the 1996 summer Olympics. I was in my office one evening, browsing around the internet, and saw a banner ad promoting NBC's TV coverage of the summer Olympics. I did exactly what the advertiser hoped I'd do: I didn't click on the ad, but instead I turned off the computer, went home, and watched the summer Olympics on NBC. But there was absolutely no way for NBC to measure or know that its ad had worked (except, of course, I've told this story in presentations at several web-advertising conferences and in various online forums).