Asterisk markers with red error-bar lines correspond to the intermediate-latitude sample, and dot markers with blue error-bar lines to the high-latitude sample. Top panel: median S/N of P_S as a function of smoothing radius. The error bars correspond to the 68% percentile of the distribution. Bottom panel: mean Spearman r-coefficient derived from a distribution using mock data as a function of the smoothing radius. The 1σ error bars correspond to propagated observational uncertainties.