A Simple Method to Check the Reliability of Annual Sunspot Number in the   Historical Period 1610-1847
  A simple method to detect inconsistencies in low annual sunspot numbers based on the relationship between these values and the annual number of active days is described. The analysis allowed for the detection of problems in the annual sunspot number series clustered in a few specific periods and unambiguous, namely: i) before Maunder minimum, ii) the year 1652 during the Maunder minimum, iii) the year 1741 in Solar Cycle -1, and iv) the so-called "lost" solar cycle in 1790s and subsequent onset of the Dalton Minimum. 
J. M. Vaquero, R. M. Trigo, M. C. Gallego
astro-ph.SR