Distribution in the significance of the EW_J0390 measurements as a proxy for calcium absorption in white dwarfs. Top panel: cumulative distribution function (CDF) of the EW_J0390 significance. The solid red line is the CDF of a normal distribution normalized to the non-DZ population. Bottom panel: color (g − i) as a function of the EW_J0390 significance. Symbols are the same as in Fig. 1. The dotted line marks zero and the gray area shows the ±2σ interval.