Despite the fact that spending on digital video ads is expected to nearly triple from $5.2 billion in 2014 to $14.8 billion 2019, with mobile making up an ever-growing share of that growth, marketers still struggle to develop the most effective mobile content.The average user increased their mobile video viewing time by 36% from 2014-2015, compared to a 3% decrease in TV viewing during the same time period. Additionally, 58% of video ads are now served across device.Those campaigns that used a hybrid of mobile and PC exposures saw increases in key metrics like brand and ad awareness, and brand favorability. The data comes via a new study by Yahoo in partnership with Nielsen and Hunter Qualitative.When it comes to mobile devices, making sure that videos are optimized for the orientation of the screen is critical. Brand lift generated from viewing an ad in the appropriate orientation was as high as 80%. Horizontal ads are more effective at generating purchase intent and increasing familiarity.It would be impossible to talk about mobile video without mentioning millennials.The study recommends shifting the emotional tone of content depending on the intended outcome: comedic tones familiarized them with the brand, dramatic and emotional ads appeal to them, and informational ads make them more likely to purchase.Fifteen-second ads were more effective than 30-second ads in native video formats. Auto-start video ads in native environments achieve 51% higher aided recall, 10% higher brand familiarity, and 4% higher affinity.