  We present time series measurements of chromospheric activity for more than
2600 main sequence and subgiant stars on the California Planet Search (CPS)
program with spectral types ranging from about F5V to M4V for main sequence
stars and from G0IV to about K5IV for subgiants. The large data set of more
than 44,000 spectra allows us to identify an empirical baseline floor for
chromospheric activity as a function of color and height above the main
sequence. We define $\Delta S$ as an excess in emission in the Ca II H\&K lines
above the baseline activity floor and define radial velocity jitter as a
function of $\Delta S$ and \bv\ for main sequence and subgiant stars. Although
the jitter for any individual star can always exceed the baseline level, we
find that K dwarfs have the lowest level of jitter. The lack of correlation
between observed jitter and chromospheric activity in K dwarfs suggests that
the observed jitter is dominated by instrumental or analysis errors and not
astrophysical noise sources. Thus, given the long-term precision for the CPS
program, radial velocities are not correlated with astrophysical noise for
chromospherically quiet K dwarf stars, making these stars particularly
well-suited for the highest precision Doppler surveys. Chromospherically quiet
F and G dwarfs and subgiants exhibit higher baseline levels of astrophysical
jitter than K dwarfs. Despite the fact that the \rms\ in Doppler velocities is
correlated with the mean chromospheric activity, it is rare to see one-to-one
correlations between the individual time series activity and Doppler
measurements, diminishing the prospects for correcting activity-induced
velocity variations.
