Let me explain it a little differently with a hypothetical scenario. You set up an ad campaign with an audience of 100k people who are already on your mailing

Let me explain it a little differently with a hypothetical scenario. You set up an ad campaign with an audience of 100k people who are already on your mailing list. You set up a conversion pixel and run the ads. Let's say that Google's tools say that 10% of people who saw the ad converted. That's incredible! But considering that they were already on your mailing list, maybe they were going to buy anyways - or maybe your TV ad is actually what convinced them to buy. You test for this by creating a holdout group from that 100k. Let's say 5k people. These are people that could have seen your ad, but we're intentionally not going to show it to them. We'll still use the conversion pixel's measurement functionality on them, and we see that 8% of those 5k people buy your product. Now, we have a clearer picture - the Google ads drive 2% more conversions for your audience - or people on your mailing list who see an ad on Google are 25% more likely to convert than those who don't.