Ratio of single-to-multiple planet systems detected with Kepler as a function of the probability that the host star is in a high-density (P_high, red) or low-density (P_low, blue) environment. The crosses show the median single-to-multiple ratio at each P_high and P_low from 1000 Monte Carlo samples of the data drawn using Poisson statistics from the observed multiplicity. The error bars show the 16th to 84th percentile range of the Monte Carlo samples. The red, opacity-weighted circles show single Monte Carlo realizations for the high-density sample. The vertical dotted line shows the fiducial threshold value of P = 0.84 used to assign systems to overdensities or the field.