Linear regression (black line) of I₆[aa] integrated over the descending phase of each solar cycle versus the integrated sunspot area over the following cycle. The expected values of integrated sunspot area for SC 25 are represented by the red circle (confidence level is obtained as root mean square error). Green circle represents the SC 24 forecasted at the end of SC 23 by using the regression from Diego et al. (2010) (represented by the green line) over SCs from n. 14 to 23.