Viewability, and Why Served Is Not the Same as Seen

Viewability is whether an advertisement was actually on a reader's screen, as opposed to merely delivered to the page. The industry test is 50 per cent of the ad in view for one continuous second.

The distinction exists because those two things come apart constantly. An ad two screens below the fold, on a page somebody closes after one paragraph, was delivered. The tag fired, the file was requested, the counter went up. Nobody saw it.

The actual standard

The Media Rating Council and the IAB set the benchmark most of the industry quotes. It is more specific than people assume.

Format	How much in view	For how long
Display, standard sizes	50 per cent of pixels	One continuous second
Display, large units of 242,500 pixels or more	30 per cent of pixels	One continuous second
Video	50 per cent of pixels	Two continuous seconds

Continuous is the word doing the work. An ad that scrolls into view, out again, and back does not add the two halves together.

Large units get a lower percentage for a sensible reason: a leaderboard that is a third on screen is genuinely readable, while a small square at 30 per cent is a sliver.

What actually happens, in order

Every step can happen without the next one. Most billing systems stop at the first.

Measurement runs in the reader's browser. A loop recalculates how much of the ad overlaps the visible part of the window, and once that crosses the threshold it starts a timer. If the ad scrolls back out, the timer resets to zero. It has to hold the position without a break.

Why a publisher should care about this

The usual framing is that viewability is something big advertisers demand and publishers grudgingly measure. For a publisher selling their own space it is more useful than that, in three specific ways.

• It is a sales argument. Telling an advertiser their money only moves when the ad was on screen is a stronger opening than any rate you could quote, and most sites selling their own inventory cannot say it at all.
• It tells you which slots are worth selling. A position that serves a lot and views little is too far down the page. That gap is a layout problem you can see and fix, rather than a mystery about why an advertiser did not renew.
• It stops you pricing a footer like a header. Once you are counting views rather than deliveries, an above the fold placement and a below the fold one stop looking like the same product. See above the fold and how to price ad space.

Where the numbers come from matters

Almost every viewability measurement, including ours, runs in the reader's own browser. That is honest measurement rather than adversarial measurement, and it is not the same thing as an independent verification vendor.

For a publisher selling five figure annual deals to a media agency, that difference will eventually come up and the answer is to buy verification. For a publisher selling a sidebar to a software company for a few hundred a month, it will not, and billing on confirmed views is already further than the buyer expected anybody to go.

The number you will see

Published viewability rates for display advertising vary enormously by site, placement and measurement vendor, and anybody quoting you a single industry figure is rounding off a very wide distribution. What is reliably true is the shape: positions above the fold measure far better than positions below it, and the gap between your best slot and your worst is usually larger than the gap between your site and the average.

Which is why the useful thing to do with viewability is not to benchmark yourself against the industry. It is to rank your own slots against each other, then price and sell them accordingly.