Is Netflix putting a dent in premium cable subscriptions? A new study suggests that a growing number of consumers are opting to pay for standalone Internet video services instead of subscribing to HBO or Showtime — although the data does not prove the trend. Total U.S. households that subscribe to HBO, Showtime, Starz and other premium TV channels declined by 6 percentage points over an 18-month span, from 38% in March 2012 to 32% in August 2013, according to a report from research firm NPD Group. Over the same period, subscriptions to Netflix and other subscription video-on-demand services including Hulu Plus and Amazon Prime Instant Video rose 4 percentage points, from 23% to 27%. One big caveat to this report: The data does not demonstrate cause and effect.