The point you're talking about ROAS/incremental ROAS is quite moot actually. Every online ad tracks the user from the moment they click the ad to whatever events they make (e.g. add to cart, purchase, etc). So the measured ROAS is exactly for the users who'd come via the ad and not any others.
I don't want to be offending but this is honestly a very basic point, of course one would only measure the Return On Ad Spend from the user's acquired by said ad and not any others... Advertisers are familiar with all such basic statistics and Google/FB ads give you very easy tools to track any person who clicks on an ad throughout their entire journey.
Edit: I was mistaken as described in the comments below
Yes, obviously ROAS is only computed over the people who saw the ad. That's not in question. The point is that knowing how many people saw the ad and then bought the product doesn't actually tell you how effective the ad was at improving your business.
Imagine a product, let's call it Oxygen, that every single person buys $10 worth of every month. It has 100% market penetration.
One day, Oxygen Corp decides to take out a $1M ad buy. They reach 1 million people, all of whom then go on to buy $10 of Oxygen, as measured by cross-site conversion tracking. $1M ad buy to move $10M of product - that's a whopping 10.0 ROAS. Must be the most effective ad campaign ever run, right?
I see what you mean and stand corrected; I wasn't assuming the case where the product sells without advertising at all.
From a gut feeling I can say that the customer came from some result of previous advertising (as we're talking about products like sodas not life-essentials like oxygen), but I guess there's no way to know since those previous ads being tv/print ads were not tracked.
If we suddenly stopped all other forms of advertising and only used online ads, in 10-20 years each customer can be tracked exactly to what ad created the first impression about the brand and thus be more accurate (still excepting marketing like word of mouth though)
You can run that same study with a control group, and sophisticated advertisers do. Show some of the users ads for your product, and other users ads for something unrelated, and compare their purchases of your product.
You are not understanding the point. You cannot actually measure incremental ROAS with basic statistics and easy Google/FB tools, because you would need to know how much they would have spent WITHOUT being exposed to the ads.
I don't know what lift tests are. It should be easy enough to devise a tool for measuring incremental ROAS under the following assumptions: a) all purchases happen online; b) I have a complete history of ad impressions and ad clicks for each user who made a purchase
As you said yourself, b) breaks down for multiple platforms. It also breaks down if users have disabled tracking.
Well, if we are talking about a recently formed company who (1) doesn't do any offline advertisment and (2) only does fully tracked online ads, you can calculate ROAS fully right? (The only exception would be word of mouth sales)
It's a big exception; otherwise you are just assuming that the only way people can get to your product is through ads. But this is equivalent to assuming "incremental ROAS = ROAS". So you propose to solve the problem by ignoring it. ;-)
PS: Of course that works if the difference is indeed small enough in your case so that it CAN be ignored.
I don't want to be offending but this is honestly a very basic point, of course one would only measure the Return On Ad Spend from the user's acquired by said ad and not any others... Advertisers are familiar with all such basic statistics and Google/FB ads give you very easy tools to track any person who clicks on an ad throughout their entire journey.
Edit: I was mistaken as described in the comments below