How is the yearly mean sunspot number calculated?
The yearly average sunspot number is now obtained by taking the average of all daily numbers for the corresponding year. The result can thus be slightly different from a simple average of the 12 monthly mean values. Indeed, the months have different lengths.
For years before 1849 and in particular before the 19th century, the yearly mean becomes more approximate. Indeed, when collecting past historical observations, R. Wolf had to derive monthly and yearly averages when sunspot counts were not available every day. For that period, the yearly average was then often obtained by averaging the monthly means.