Because some people have more than one phone line, they end up being double-counted. This results in overstated audience figures and penetration rates that approach and eventually exceed 100% of the population. For marketers, the number of mobile users is a more useful figure because it more accurately describes the audience, and thus potential reach.
How big is the actual US mobile audience? Taking into account people of all ages, eMarketer estimates that mobile penetration is 76.5% in 2009, or 235 million people. This is expected to rise gradually to 255.4 million in 2013, or 80% penetration.
Predictably, this estimate is much lower than figures from CTIA â€“ The Wireless Association, which measured 276.61 million mobile subscribers (subscriptions) as of June 2009. Using data from the US Census Bureau, this equates to 90% of the total US population.