Note: This post was co-authored by Kris Harbold and Andy Chen.

By looking at the amount of time visitors are exposed to ads on different parts of a web page, we can get a sense of how much value your ad inventory generates for advertisers. And the results might not be what you expect.

It turns out, traditional advertising heuristics about which parts of the page are most valuable are wrong. Let’s look at figures 1 and 2, which show us the relationship between where the middle of an ad is positioned and how it’s viewed. We look at how ads are viewed through two metrics: viewability and average active exposure time. An ad is viewable if at least 50% of the area is in view for at least one continuous second. Of those visitors that have the opportunity to view the ad, average active exposure time is how many seconds they’ve spent actively browsing the page while that ad is in view.

As we might expect, both viewability and average active exposure time eventually trend downward. But this broad downward trend isn’t without a few blips. In particular, let’s consider the conspicuous dip in both viewability and average active exposure in the topmost 500 pixels of the page. Notice that viewability and average active exposure drop to a level that isn’t reached until 1,500 and 2,500 pixels down, respectively. This can be explained by the fact that visitors tend to start scrolling down the page right after they arrive, sometimes even before the page fully loads. Then, visitors settle on engaging content further down the page, which results in the subsequent increase in our metrics.

So what does this mean for your inventory? We know that the value of an ad position depends not only on its viewability but also on average active exposure. It follows that the most valuable ad real estate isn’t at the very top of the page, but rather just below the top, where both our metrics peak out. And while both metrics decrease below the fold, we see that average active exposure is remarkably resilient: a visitor that sees an ad 2,500 pixels below the top will still spend an average of 10 seconds of engaged time with the ad in view. That’s plenty of time for the ad to have an effect on the viewer.