The referrer field is very unreliable for these purposes. Extremely unreliable. First of all, the referrer doesn't even get recorded a lot of the time. Second, the sites displaying PPC ads, whether search engines or otherwise, can show up as all sorts of things that may or may not suggest that a PPC engine served the ad or directed the request. Don't even try to sort it out, it is a waste of time unless you don't mind really rough estimates.
The only way to get close approximations of your PPC traffic is to put marker parameters in the destination URL that's programmed into the PPC ad. At the simplest level it would be something like "source=PPC_ad" although most people try to get a lot more information than that, like which PPC program, what keyword was paid for, and so on.
Then, if you have a good idea of all search engine traffic, you subtract this activity from the total SE traffic and you have an estimate of organic SE traffic. This, however, requires 1) that the referrer field be populated, which it often isn't, and 2) that you have an accurate lookup table that contains a complete list of referrer strings that belong to search engines as opposed to non-search engines.
Thank you for the thoughts. Placing markers would unfortunately be impossible. This service is strictly to help our clients sort their ROI from organic and PPC search engine visibility. We are not placing or in control of their PPC.
The final stage of the program will be to track the customer from a search engine to the website and on to filling out a form on the site.
Any additional thoughts on distinguishing PPC refers from organic?
Referrers from GG, Yahoo, and MSN look the same for organic search as for PPC. Frequently there is no referrer. If you are showing ads on content with GG occasionally the referrer begins with: [pagead2.googlesyndication.com...]
Using the referrer field to distinguish between them is very unreliable.
I would advise my clients to use a specific parameter in the destination url and track it as a unique channel in the software. This simple solution works for PPC, email, and other offline advertising.