A Note on the Relationship between Sunspot Numbers and Active Days
  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. 
J.M. Vaquero, S. Guti\'errez-L\'opez and A. Szelecka
astro-ph.SR