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Incrementality is the only metric that matters
You want ads to sell stuff to people who are buying a competitor's product, or who don’t know that your product exists.
To see if your ads change behavior, compare the conversion rate of a group that saw your ad vs. a group that did not.
If the conversion rate of the group that saw your ad is higher, the ad created behavioral change.
Spending more on ads that induce behavioral change will generate incremental revenue, but the above example is oversimplified. Figuring out which ads create incremental revenue isn't simple, especially for brands spending 8+ figures on ads across multiple channels! Start here for more practical examples, or keep scrolling.
Incremental lift is a measure of advertising effectiveness.
Incremental lift measurement can be applied to any type of media, in any stage of the funnel. Especially when evaluating performance marketing, practitioners tend to focus on measures of efficiency and volume. In other words, we try to spend spend the whole budget, within the allowable cost per conversion. This is understandable - platforms are incentivized to facilitate the collection and reporting of data through this lens - but problematic. Looking at performance this way ignores whether or not a sale would have happened regardless of ad exposure. Measuring lift will steer your budget to ads that are effective at creating behavioral change.
If you rely on any kind of platform reporting (even with rules for attribution), your ads are claiming credit for sales that would have happened regardless of ad exposure.
Selection bias in algorithmic media buying is the baseline problem here. The machine you feed your budget into generally has two priorities (in order of importance): 1. Spend the entire budget; 2. Minimize cost per goal (see the above, max volume while controlling for efficiency). Convincing a prospect to change their behavior is much more expensive than taking credit for a conversion because you served a banner ad below the fold to someone on your email marketing list. Stimulating behavioral change creates incremental value for your business; buying banner ads like they are billboards on the highway, and claiming credit for every user that figuratively drives by on the road to purchase, simply makes your ad budget disappear. Do the first thing.
incremental lift measurement playbook
It's possible that the ads you buy make no meaningful contribution to sales, and only claim credit for things that would have happened regardless of spending money on ads.
A quick review of how to piece together lift measurement by channel.
Display, especially programmatic display, is a crapshoot. Retargeting is useless. Unless you are more sophisticated than the professional arbitrageurs and bot networks (you aren't), save your money.
Brand search is extortion; non-brand has limited utility and is ridiculously expensive.
Between Facebook and the 'Gram, you can basically reach every human with a wallet on the planet, on demand.
It still works.
The one in which I describe a possible privacy crime. Use ads to upsell to current customers - this might be the best opportunity for large scale businesses to create incremental revenue with advertising.