Measured slope of the IMF as a function of metallicity (top panel), [Mg/Fe] (middle panel), and stellar surface mass density (bottom panel). In each panel, symbols are color coded according to age (top), stellar surface mass density (middle), and total metallicity (bottom). Measured IMF values appear to correlate with the three quantities, with the IMF becoming increasingly steep for more metal-rich, less [Mg/Fe]-enhanced, and denser regions in NGC 3351.