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The method of active day fraction (ADF) was proposed recently to calibratedifferent solar observers to the standard observational conditions. The resultof the calibration may depend on the overall level of solar activity during theobservational period. This dependency is studied quantitatively using data ofthe Royal Greenwich Observatory, by formally calibrating syntheticpseudo-observers to the full reference dataset. It is shown that the sunspotgroup number is precisely estimated by the ADF method for periods of moderateactivity, may be slightly underestimated by 0.5\,--\,1.5 groups ($\leq$10\,\%)for strong and very strong activity, and is strongly overestimated by up to 2.5groups ($\leq$30\,\%) for weak--moderate activity. The ADF method becomesunapplicable for the periods of grand minima of activity. In general, the ADFmethod tends to overestimate the overall level of activity and to reduce thelong-term trends.

A Test of the Active Day Fraction Method of Sunspot Group Number Calibration: Dependence on the Level of Solar Activity