Abstract. We present a comparison of electron densities derived from optical forbidden line diagnostic ratios for a sample of over a hundred nebulae. We consider four density indicators, theand [Ar ] λ4711/λ4740 doublet ratios. Except for a few H  regions for which data from the literature were used, diagnostic line ratios were derived from our own high quality spectra. For the [O ] λ3729/λ3726 doublet ratio, we find that our default atomic data set, consisting of transition probabilities from Zeippen (1982)