Major change of data set on July 1st, 2015
Over the past 4 years a community effort has been carried out to revise entirely the historical sunspot number series. A good overview of the analyses and identified corrections is provided in the recent review paper: Clette, F., Svalgaard, L., Vaquero, J.M., Cliver, E. W., "Revisiting the Sunspot Number. A 400-Year Perspective on the Solar Cycle", Space Science Reviews, Volume 186, Issue 1-4, pp. 35-103. This is actually the first deep revision of the sunspot number since its creation By R. Wolf in 1849.