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We’re more than 15 years into the age of streaming media, and yet we still don’t know the answer to two basic questions: How many people are watching, and who are they?

A recent study by GroupM and ad measurement firm iSpot.tv has renewed questions about the audience for ads on connected TVs and how to measure it. The study found that about 17% of advertisements shown on televisions connected through a streaming device—including streaming boxes, sticks and gaming consoles—play while the TV is turned off. The study also found that on average, between 8% and 10% of all streaming ads were shown while the TV was off. All were considered “impressions” for which advertisers were billed, but only sensors inside the TV detected the ads. No person saw them.

This is a fixable problem. The Wall Street Journal reported that both iSpot.TV and the Interactive Advertising Bureau are building products to verify that TVs are on when commercials are delivered.