  We have studied the relationship between three different versions of the
sunspot number (Group, International and American sunspot number) and the
number of active days (i.e., the number of days with spots on the solar disk).
We have detected an approximately linear relationship for low solar activity
conditions. However, this relationship for the International sunspot number is
very different to the ones obtained with the other versions of the sunspot
number. The discordant values correspond to older observations.
