Smartphones are being adopted at a phenomenal pace but
little is known (publicly) today about how people use these
devices. In 2009, smartphone penetration in the US was 25%
and 14% of worldwide mobile phone shipments were smartphones
[23, 16]. By 2011, smartphone sales are projected to
surpass desktop PCs [25]. But beyond a few studies that report
on users’ charging behaviors [2, 17] and relative power
consumption of various components (e.g., CPU, screen) [24],
many basic facts on smartphone usage are unknown: i) how
often does a user interact with the phone and how long does