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We identify several pairs of 'equivalent' observers defined as observers withequal or nearly equal 'observational threshold' areas of sunspots on the solardisk as determined by the 'Active Days Fraction' method [e.g. Willamo et al.,2017]. For such pairs of observers, the ADF-method would be expected to map theactually observed sunspot group numbers for the individual observers to tworeconstructed series that are very nearly equal and (it is claimed) represent'real' solar activity without arbitrary choices and deleterious,error-accumulating 'daisy-chaining'. We show that this goal has not beenachieved (for the critical period at the end of the 19th century and thebeginning of the 20th), rendering the ADF-methodology suspect and not reliablenor useful for studying the long-term variation of solar activity.

Assessment of the Failure of Active Days Fraction Method of Sunspot Group Number Reconstructions