Actually, there is no exception.

After I published my previous post on the use of control groups, I did a google search on the use of control groups, and came up with the following quote:

The cardinal rule of direct marketing is to include a control group. Without it, you will never know whether customers purchased your product because of this marketing effort, or because of the billboard ad, the radio spot, a friend’s suggestion, an in-store brochure, or because Elvis told them to. There is one exception. If you have an air-tight fulfillment set up, whereby the customer can only purchase the product through your channel, e.g. a special 800#, then you don’t need to hold out a sample; you can be certain that every sale came from your effort (except the referrals from Elvis).

The conclusion is incorrect. I have seen more campaigns than I care to count on all of my fingers that got a ton of calls to the 1-800 number, but no incremental sales whatsoever. Zero. Zilch. Ноль.

Sales that come to the 1-800 number is something that I like to call "associated sales", i.e. sales that came from the target audience and somehow "associated" with the communication, but they are is not the same as incremental sales. Your mail piece may be very good an convincing potential customers who would make a purchase anyway to call a particular phone number, but that's no proof they would not have called if not for the DM piece. They would have called alright (that's what the control group is for!), just called a different number.

I don't like the 1-800 phone number salesmen much. Oh, well, comes with the territory (and I do believe in Elvis referrals).