Observed distribution of the ridge-line-detrended δ₄ index for stars brighter than y₀ = 19 mag (grey histogram) and two different models for the intrinsic distribution. Red vertical lines represent a sampling from the posterior pdf of the location of the two populations with no intrinsic spread in the mixture model (Eq. (10)). Blue transparent curves show a sampling from the posterior pdf of the intrinsic Gaussian distribution model. Best-fitting values for the parameters of each model in terms of the median of the posterior and 68 per cent credible intervals are shown at the top. The solid red and blue lines correspond to the best respective models convolved with the photometric uncertainties and integrated over the bins of the histogram. A leave-one-out cross-validation to quantify which model fits better yields relative odds of ~5.1 to one for the mixture and Gaussian models (see text).