  The Kepler Mission is searching for Earth-size planets orbiting solar-like
stars by simultaneously observing >160,000 stars to detect sequences of transit
events in the photometric light curves. The Combined Differential Photometric
Precision (CDPP) is the metric that defines the ease with which these weak
terrestrial transit signatures can be detected. An understanding of CDPP is
invaluable for evaluating the completeness of the Kepler survey and inferring
the underlying planet population. This paper describes how the Kepler CDPP is
calculated, and introduces tables of rms CDPP on a per-target basis for 3-, 6-,
and 12-hour transit durations, which are now available for all Kepler
observations. Quarter 3 is the first typical set of observations at the nominal
length and completeness for a quarter, from 2009 September 18 to 2009 December
16, and we examine the properties of the rms CDPP distribution for this data
set. Finally, we describe how to employ CDPP to calculate target completeness,
an important use case.
