Illustration of the effect of noise in the Hα image on the NCR distribution. The distributions with the dashed lines were obtained using an S/N approximately five times better than that of the A12 images; the solid lines represent the A12 S/N; the dotted lines represent a five times worse S/N. The main effect is an increase in the number of zero-NCR stars, and this effect is most significant on initially low NCRs.