Originally posted on RapidTVNewsThe survey shows that advertisers spent $7.5 billion on video advertising in 2015, a 42% growth over 2014, as the share of impressions that Viant's advertisers served onto computer devices fell from 88% of the company's total impression delivery to 79%, down 9%.Also, nearly a fifth of all impressions (18%) were served to smartphones and tablets, up 59% from the previous year, when mobile accounted for 11% of the total.But making the biggest gains were over-the-top (OTT) television-based watching, which grew six-fold in share, from 0.5% of the total in 2014 to 3% in 2015. At its current growth trajectory, OTT seems to on track to overtake tablets for share of digital ads in 2016.In its 2014 report, the company noted that more video ads were being served as 15-second than 30-second clips, compared to 2013 when the longer ads were in the majority. In 2015, that trend seems to have reversed, and 30-second ads are making a comeback to roughly an even share compared to 15-second ads. That trend holds true across all publisher types."We speculate this may be related to the continued pace of cord-cutting and the growth of over-the-top TV content consumption, where people are giving up their cable subscriptions and watching those same shows over Internet connections on various devices," the report noted. "People watching more long-form video content via the Internet would coincide with a shift back to longer format ads in the digital ad-serving mix."As with last year, the highest completion rates for video ads by device type were on TV-based devices, namely OTT and game consoles, where nearly 100% of digital video ad views were allowed to run to completion. The company speculates this has much to do with the 'lean back' nature of the TV device and perhaps the awkwardness of ad-skipping with a remote control.Viant's report also found that viewability rates are flat: after two years of industry focus on improving the 'viewability' rate of digital ads, with the IAB advocating last year for 70% averages for campaigns, and the 4A's insisting 100% should be the standard, the actual rate was only 46%, effectively unmoved from the year prior.Major media brands lead in viewability and completion rates. The digital properties of traditional media companies (eg, CBS.com, CNN.com, People.com) saw improvements over last year in key performance indicators for digital ad campaigns and had viewability and video ad completion rates (71% and 92%, respectively) that were far higher than the media categories of Internet brands and ad exchanges/networks.And finally, click-through rates, the perennial ad metric everyone loves to hate, fell significantly for video ads, from 0.62% on average in 2014 to 0.43% in 2015. This may suggest that advertisers are recognising digital video primarily as a branding tool.