Basic properties of the sample of SSUBs, inferred from Ca ii H data. The left and centre panels plot histograms of peak length and lifetime, respectively, with the dashed lines in each frame corresponding to the appropriate mean values. The right panel displays a scatter plot of peak length against lifetime, with the dashed line indicating a least squares regression linear fit calculated for these data (with a rate of change of an extra 0.0006′′ of length per extra second of lifetime). The triangle plots the position of the mean of both variables.