Are you running the incrementality study, or is your provider (e.g. Google)? Because one of the points here is that providers also have an incentive to fudge the numbers - they're not in business of being accurate, they're in business of making it seem their services help you.
To riff off the average height example - you can get a pretty darn good estimate based on sampling and good statistical practice... unless the people doing the sampling have a reason to bias the measurement, or - if you're sampling on your own - people who control where you go and who you meet...
Oh that could definitely be the case, although I'll note that the larger actors have more to lose and are at more risk of being caught for engaging in fraud.
I think my usage of "you" was confusing because I work in sell-side advertising.
You nailed this. Tirole discusses this in his paper hierarchy on bueracracies. Your internal stakeholders aren't always aligned to the company, sometimes it is to a vendor.
To riff off the average height example - you can get a pretty darn good estimate based on sampling and good statistical practice... unless the people doing the sampling have a reason to bias the measurement, or - if you're sampling on your own - people who control where you go and who you meet...