The study investigates the impact of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity (MZ) relation using 27,730 star-forming galaxies from the SDSS DR4. The results show that the choice of metallicity calibration significantly affects the shape and y-intercept of the MZ relation. The absolute metallicity scale varies up to 0.7 dex depending on the calibration, and the change in shape is substantial. The study presents new metallicity conversions that allow metallicities derived from different strong-line calibrations to be converted to a common base calibration, reducing discrepancies between calibrations and achieving agreement in the MZ relation to within 0.03 dex on average. The study also finds that AGN classification methods have negligible effects on the SDSS MZ relation. The turn-over of the MZ relation at M* ~ 10^10 M_sun depends on aperture covering fraction, and a lower redshift limit of z < 0.04 is insufficient for avoiding aperture effects in fiber spectra of the highest stellar mass galaxies. The study highlights the importance of using the same metallicity calibration when comparing different MZ or luminosity-metallicity relations. The results show that the MZ relation is steep for masses < 10^10.5 M_sun and flattens at higher masses. The study also finds that the MZ relation is affected by the aperture covering fraction, with a lower covering fraction leading to significant discrepancies between fixed-sized aperture and global metallicity estimates. The study concludes that the MZ relation is robust when using the same metallicity calibration and that the new conversions successfully remove the large discrepancies between calibrations. The study also finds that the MZ relation is affected by the AGN classification methods, with different schemes leading to different results. The study recommends using the new conversions to convert metallicities from one calibration to another and to ensure consistency in the MZ relation. The study also finds that the MZ relation is affected by the stellar mass and that the scatter in the MZ relation is large for all calibrations. The study concludes that the MZ relation is a crucial tool for understanding galaxy formation and evolution, and that the results have important implications for the study of galaxy evolution.The study investigates the impact of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity (MZ) relation using 27,730 star-forming galaxies from the SDSS DR4. The results show that the choice of metallicity calibration significantly affects the shape and y-intercept of the MZ relation. The absolute metallicity scale varies up to 0.7 dex depending on the calibration, and the change in shape is substantial. The study presents new metallicity conversions that allow metallicities derived from different strong-line calibrations to be converted to a common base calibration, reducing discrepancies between calibrations and achieving agreement in the MZ relation to within 0.03 dex on average. The study also finds that AGN classification methods have negligible effects on the SDSS MZ relation. The turn-over of the MZ relation at M* ~ 10^10 M_sun depends on aperture covering fraction, and a lower redshift limit of z < 0.04 is insufficient for avoiding aperture effects in fiber spectra of the highest stellar mass galaxies. The study highlights the importance of using the same metallicity calibration when comparing different MZ or luminosity-metallicity relations. The results show that the MZ relation is steep for masses < 10^10.5 M_sun and flattens at higher masses. The study also finds that the MZ relation is affected by the aperture covering fraction, with a lower covering fraction leading to significant discrepancies between fixed-sized aperture and global metallicity estimates. The study concludes that the MZ relation is robust when using the same metallicity calibration and that the new conversions successfully remove the large discrepancies between calibrations. The study also finds that the MZ relation is affected by the AGN classification methods, with different schemes leading to different results. The study recommends using the new conversions to convert metallicities from one calibration to another and to ensure consistency in the MZ relation. The study also finds that the MZ relation is affected by the stellar mass and that the scatter in the MZ relation is large for all calibrations. The study concludes that the MZ relation is a crucial tool for understanding galaxy formation and evolution, and that the results have important implications for the study of galaxy evolution.