(a) The fit of a regression line (solid line) with the 95% confidence bands (dashed lines) to the histogram points of number of sunspot groups creating one cluster from the northern hemisphere in the log-linear scale plot for (a) cycle 12, (b) cycle 13, (c) cycle 14. (d–f) The same as in the upper row, but for cycles 15–17. (g–i) The same as in the upper row, but for cycles 18–20. (j–l) The same as in the upper row, but for cycles 21–23. The correlation coefficients and the regression equations are presented in the left lower corners.