Monte Carlo test. Left: PDFs with different cuts. PDFs are normalised so that the area below each curve corresponds to 1. We used these PDFs to randomly extract values to add to the radio power of halos on the correlation and distributed them on the M–P plane. Right: example random distribution of clusters in the mass–radio power diagram. Errorbars are radio halos, arrows are upper limits. The black and red solid lines represent the radio halo and upper limits correlation we find in the sample, respectively.