Density distribution of the difference between inferred and true [Fe/H] at galactic center separated by top: lookback time τ and bottom: bar strength. The projected [Fe/H](R = 0 kpc, τ) is estimated from the [Fe/H] value at a given radius and the current [Fe/H] gradient for a given age of width 1 Gyr (see Figure 7). As a whole, the value of [Fe/H](R = 0 kpc, τ) is underestimated by ∼0.2 dex, with larger differences for weaker barred galaxies and larger lookback times. The bias towards negative values (i.e. [Fe/H](R = 0 kpc, τ) is underestimated) is predominantly due to the non-linear spike in [Fe/H] in the inner kpc (see Figure 7 for an example).