(a) A plot of Hoyt & Schatten (1998a) k′-factors vs. the assigned number of the associated observer and the year that this observer began recording sunspot groups. Blue and red data points indicate observers who recorded sunspot groups for <100 and ≥100 days, respectively. The time intervals and first and last observers for each of the four segments separated by the dashed vertical lines are as follows: (1) 1700–1840 (J. Cassini to Lohse); (2) 1841–1883 (Schmidt to Merino); (3) 1884–1920 (Kokides to Batavia); and (4) 1921–1992 (Mt. Wilson (Full Disk) to British Astronomical Association). Dashed horizontal lines give average values of the k′-factors for observers who began counting groups during each interval (i.e., “new observers” during that interval). The slanted solid line in interval 3 indicates the increase in k′-factors for new observers during this period. Black circles indicate the k′-factors of Wolf (No. 298) and Wolfer (No. 338). The mean uncertainty in k′ for Observers 91–343 is ~0.1*k′; thereafter it is zero (Hoyt & Schatten 1998a; Appendix 1). (b) Same as (a) except here only intervals 2–4 are shown and revised k′-factors (k′*-factors) are taken from Cliver & Ling (2016). The endpoint observers for interval 4 are Mt. Wilson (Full Disk) (1923) and Dogan (1974).