This phenomenon is known as the "Organic Percentage". You execute a mail group and a holdout group, very simple testing.
|Organic Percentage: Mail & Holdout Test Groups|
|Organic %, Catalog = (3.00 - 0.50) / 3.00||83.3%|
|Organic %, Total = 14.50 / 15.00||96.7%|
This outcome is seldom if ever mentioned by e-mail vendors or catalog marketing vendors. The opposite outcome is mentioned all of the time --- we hear that catalogs and e-mails drive sales across all channels. This outcome is more common than the oft-publicized "catalogs drive sales to all channels" outcome we read about.
Almost nobody talks about reducing marketing expense and increasing profit because we are mis-attributing orders to customers who would have ordered anyway. The vendor ecosystem would be hurt if this outcome were published on a frequent basis.
In this case, 96.7% of customer demand happens anyway .. only 3.3% of the $15.00 is because of catalog marketing.
Quantifying the organic percentage is the single most important thing that catalog and e-mail marketers will do in 2009.
Acting upon the organic percentage is the single most important thing that catalog and e-mail marketers will do in 2010.
At this time, the majority of my Multichannel Forensics projects require me to greatly reduce marketing expense while keeping demand coming in at a high rate. It is likely that you'll be spending a ton of time on this issue in 2010 as well.