An online ad needs to be on the screen for 14 seconds to have any chance of being looked at, according to a new study from InSkin Media, Research Now and Sticky.

Eye-tracking reveals that 25% of ads defined as viewable - industry guidelines state that 50% of the pixels must be on screen for at least one second - are never looked at.

One third achieve a 'gaze time' (time spent actually looking at the ad) of less than a second, while only 42% are looked at for at least a second. The median time a viewable ad is actually gazed at is 0.7 seconds.

According to the study, ads achieving at least one second of gaze time are viewable for an average of 26 seconds. For at least two seconds' gaze time the average viewability is 33 seconds, while for 3+ seconds of gaze time, average viewability is 37 seconds.

"A campaign should be assessed in three stages: did the ad have the opportunity to be seen, was it actually looked at and what was the impact," said Steve Doyle, InSkin Media's CCO.

"It should be judged and optimised against the last stage (impact) but the focus on viewability means campaigns are increasingly optimised against the first stage (the opportunity) which can be counter-productive to maximising impact.

"Why? Smaller formats have higher 'opportunity to be seen' rates as their size means it's easier to hit viewability thresholds - but gaze time is very low. Thus, it's optimising on low engagement and low impact."

Doyle added that publishers must "tread the fine line" between hosting more ads which drives revenue, or less ads which mean stronger results for advertisers and a better user experience.

"It's obvious which one is most conducive to long-term loyalty from clients and readers," he said.