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Plasma diagnostics are the bases of investigation into the physical and chemical properties of line-emitting gaseous systems. To perform plasma diagnostics properly, it is essential to correct the input spectrum for extinction properly. This is simply because determining the degree of extinction is dependent on the physical properties of the line-emitting gas. Hence, both extinction correction and plasma diagnostics have to be performed simultaneously and self-consistently. By comparing the results of analyses performed for a sample of nine bright planetary nebulae in M 31 with and without the proper extinction correction and plasma diagnostics, we demonstrate how initial assumptions for the physical conditions of the line-emitting gas in extinction correction would compromise the results of the entire analyses. While the electron density/temperature are relatively immune to the imposed inconsistent assumptions, the compromised extinction would cause systematic offsets in the extinction-corrected line strengths, which consequently would impose adverse effects on the resulting ionic and elemental abundances, and other inferences made from the incorrect results. We find that this M 31 PN sample simply represents those around the high-mass end of the mass range for low-mass planetary nebula progenitor stars as expected from the existing theoretical models. It appears that the suspicion raised in the previous study - these PNe being anomalously nitrogen overabundant - is simply caused by the apparent underestimate in extinction that originates from the imposed inconsistent assumptions in extinction correction. In a larger context, the results of plasma diagnostics in the literature without seeking simultaneous self-consistency with extinction correction have to be handled cautiously. Ideally, such previous results should be re-evaluated by seeking simultaneous self-consistency.