Histogram distribution of the total square perpendicular distance (TSPD) for a linear fit to 10⁵ random samples of 10 HB stars, compared to TSPD_var, the TSPD of the fit to the RR Lyrae stars in NGC 1904 (red vertical dotted line). The cumulative probability distribution is also plotted as a dashed line; the probability that the value of TSPD is equal to or lower than TSPD_var is  ~2.8%.