  A large share of all online display advertisements (ads) are never seen by a
human. For instance, an ad could appear below the page fold, where a user never
scrolls. Yet, an ad is essentially ineffective if it is not at least somewhat
viewable. Ad viewability - which refers to the pixel percentage-in-view and the
exposure duration of an online display ad - has recently garnered great
interest among digital advertisers and publishers. However, we know very little
about the impact of ad viewability on advertising effectiveness. We work to
close this gap by analyzing a large-scale observational data set with more than
350,000 ad impressions similar to the data sets that are typically available to
digital advertisers and publishers. This analysis reveals that longer exposure
durations (>10 seconds) and 100% visible pixels do not appear to be optimal in
generating view-throughs. The highest view-through rates seem to be generated
with relatively lower pixel/second-combinations of 50%/1, 50%/5, 75%/1, and
75%/5. However, this analysis does not account for user behavior that may be
correlated with or even drive ad viewability and may therefore result in
endogeneity issues. Consequently, we manipulated ad viewability in a randomized
online experiment for a major European news website, finding the highest ad
recognition rates among relatively higher pixel/second-combinations of 75%/10,
100%/5 and 100%/10. Everything below 75\% or 5 seconds performs worse. Yet, we
find that it may be sufficient to have either a long exposure duration or high
pixel percentage-in-view to reach high advertising effectiveness. Our results
provide guidance to advertisers enabling them to establish target viewability
rates more appropriately and to publishers who wish to differentiate their
viewability products.
