This is interesting. One of the eternal questions about advertising is "would I have got those enquiries anyway?" In other words, were the responders looking for me in the first place, and just chose that advert as the way of getting in touch? Of course, with traditional business-to-business advertising, where response is barely monitored in the first place, it's not really an issue. But nowadays when return on investment can be measured down to the last decimal point, it's a far more pressing question.
Well, B2Blog recounts a recent experiment in Turn off AdWords?! What I learned… which proved – to the author's obvious pleasure – that in his company's case, the responders wouldn't have found them anyway. In other words, the investment in the advertising brought in business which wouldn't have come in otherwise. Always a relief.