OTS and LTS: DOOH Impression Metrics

OTS and LTS are two core metrics that define the quality of DOOH impressions. This article covers OTS (Opportunity-to-See) and LTS (Likelihood-to-See), the shift from OTS to LTS in IAB’s 2025 guidance, and why LTS better reflects real viewing. The aim is to clarify the measurement logic behind the question “how many people saw it.”

What is OTS? Opportunity-to-See

OTS expresses the number of people who had the opportunity to see an ad. The key word here is “opportunity.” OTS counts that a person passed through the screen’s catchment — that they could have seen the ad — without guaranteeing that they actually looked.

Out-of-home advertising was historically built on OTS, because it was the easiest thing to measure. Traffic counts, pedestrian flow data, and mobile location answer “how many people passed this point” relatively easily. But OTS has an obvious weakness: a person passing near the screen does not mean they turned their head and looked. Someone walking past sideways, looking at their phone, or moving away from the screen is still included in OTS. As a result, OTS tends to overstate real viewing.

OTS is also the classic basis of impression counting; how many people a play “reached” has most often been calculated with OTS logic.

What is LTS? Likelihood-to-See

LTS measures not just whether a person had the opportunity to see the ad, but the likelihood that they actually saw it. LTS filters the raw passing count through a set of correcting factors: the person’s position and angle relative to the screen, how long they stayed within the screen’s field of view, the screen’s size and height, and the surrounding visual competition.

It helps to think of the logic as a funnel. The widest layer is everyone who passed through the screen’s catchment. A subset of them genuinely had the opportunity to see (OTS). A subset of OTS, in turn, carries a high likelihood of seeing because the conditions were truly favourable (LTS). At the very bottom is measurable real attention; we cover how attention is measured and its limits in the attention measurement article.

LTS always produces a smaller number than OTS, because it is more selective. That is not bad news; on the contrary, it is a more honest estimate of what the advertiser actually gets.

Why is the IAB moving from OTS to LTS?

One of the clearest directions in DOOH measurement in recent years is the industry’s shift from OTS to LTS. IAB’s 2025 DOOH measurement guidance explicitly reflects this shift. The rationale is the increasing commercialization of measurement.

As advertisers buy programmatically and on a currency basis, the answer to “how many people did I actually reach” becomes more critical than “how many people passed by.” When OTS’s tendency to overstate combines with the confidence gap between modeled and measured audiences, the need arises for a more conservative, more realistic metric. LTS fills that gap. You can see the roles of the IAB, the MRC, and currency providers that shape the standard holistically in the audience measurement overview.

This shift is also compatible with camera-based measurement. When a screen-side sensor can observe whether a person is oriented toward the screen (head-pose) and how long they stayed (dwell), LTS no longer has to be merely modeled; it can be supported by directly measured data. That makes the metric more defensible.

OTS and LTS compared

Placing the two metrics side by side clarifies the difference.

Dimension	OTS (Opportunity-to-See)	LTS (Likelihood-to-See)
What it measures	Opportunity to see	Likelihood of actually seeing
Key question	How many passed?	How many likely looked?
Correcting factors	Few	Angle, dwell, screen size, visual competition
Typical magnitude	Higher (tends to overstate)	Lower (more conservative)
Reflecting real viewing	Weak	Stronger
IAB direction	Traditional basis	Emphasized in 2025 guidance

As the table shows, OTS and LTS are not rivals but two successive tiers. OTS is still a base input; LTS is a refinement layer that brings that input closer to real viewing.

How do OTS and LTS affect the impression count?

The practical effect shows up in the billable impression count. When a campaign is reported on OTS alone, the reached audience can look higher than it really is. When the same campaign is reported on LTS, a lower but more realistic impression count results. This lets the advertiser measure the return on their spend more accurately.

An important nuance: although LTS is more realistic, it is still a probability estimate, not a guarantee of viewing. “High likelihood of seeing” is not the same as “definitely saw it and paid attention.” Measuring real attention is a separate layer with its own limits. For that reason, OTS, LTS, and attention should be considered together; none gives the full picture on its own.

Summary

OTS measures the opportunity to see, while LTS measures the actual likelihood of seeing. The shift from OTS to LTS in IAB’s 2025 guidance is a refinement that brings DOOH measurement closer to real viewing and provides a sounder basis for programmatic trading — but even LTS is a probability, not a guarantee.