RV of Kepler-78 vs. time, measured by Keck-HIRES and HARPS-N. The HIRES and HARPS-N data are plotted as blue and red points, respectively, with errors in RV shown. A Keplerian orbit signal with the calculated best-fit Doppler amplitude has been subtracted from the data. The colored lines correspond to the Gaussian process regressions with best-fit kernel parameters of the correspondingly colored RV measurements, where the shaded regions correspond to 1σ and 2σ uncertainties. Both data sets have been fitted with a single quasi-periodic kernel operator with common period, roughness, and lengthscale (θ, w, and λ) hyperparameters, but separate covariance amplitude and white noise (h and σ) parameters.