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Published online by Cambridge University Press: 25 May 2016
I review the arguments favoring the high reliability of the PNLF method. Agreement with Cepheid distances is better than 8%, and consistency for multiple PNLF distances within clusters is even better. Agreement between Cepheid distances to spirals and PNLF distances to ellipticals within the same cluster also is excellent. In order for the PNLF method to work despite the vast diversity of properties seen among PN, several factors must operate. Most importantly, the progenitors of the bright extragalactic PN probably have ages less than ∼10 Gyrs. Also, very young PN must either be absent from the bright extragalactic samples or must have lower luminosities than suggested by their predicted central star masses. The latter may be due to internal dust, nitrogen enrichment that competes with oxygen for ionizing photons, and/or a tendency for massive PN to be optically thin to ionizing radiation. In addition, models that reproduce observed PNLFs rely on theoretical evolutionary tracks of central stars, and so these tracks also must be reasonably correct. PNLF observations suggest, however, that the slope of the initial-to-final mass relation is shallower than the Weidemann (1987) relation.