  We present results from spectroscopic follow-up observations of stars
identified in the Kepler field and carried out by teams of the Kepler Follow-Up
Observation Program. Two samples of stars were observed over six years
(2009-2015): 614 standard stars (divided into "platinum" and "gold" categories)
selected based on their asteroseismic detections and 2667 host stars of Kepler
Objects of Interest (KOIs), most of them planet candidates. Four data analysis
pipelines were used to derive stellar parameters for the observed stars. We
compare the $T_{\mathrm{eff}}$, $\log$(g), and [Fe/H] values derived for the
same stars by different pipelines; from the average of the standard deviations
of the differences in these parameter values, we derive error floors of $\sim$
100 K, 0.2 dex, and 0.1 dex for $T_{\mathrm{eff}}$, $\log$(g), and [Fe/H],
respectively. Noticeable disagreements are seen mostly at the largest and
smallest parameter values (e.g., in the giant star regime). Most of the
$\log$(g) values derived from spectra for the platinum stars agree on average
within 0.025 dex (but with a spread of 0.1-0.2 dex) with the asteroseismic
$\log$(g) values. Compared to the Kepler Input Catalog (KIC), the
spectroscopically derived stellar parameters agree within the uncertainties of
the KIC, but are more precise and are thus an important contribution towards
deriving more reliable planetary radii.
