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tokens: provide the best overall fits in terms of visual inspection of the residuals and through comparison of likelihood ratios between models. Thus, we adopt the BaSTI models for this Letter. We adopt a metallicity grid that ranges from −2.3 ≤ [M/H] ≤ −0.5 with a resolution of 0.1 dex and an age grid that ranges from with a resolution of dex. We find that including ages younger than did not change the SFHs (as these galaxies have no young populations) but increased the computational time. Thus, we simply exclude ages with from our CMD modeling. Finally, given that our CMDs do not reach the oldest MSTO, we follow Weisz et al. ( 2011 , 2014 ) and adopt a prior on the age–metallicity relationship that requires the metallicity to increase monotonically with time, with a modest dispersion allowed at each age. This choice helps to mitigate some of the age–metallicity degeneracy on the RGB and RC. We compute random (which accounts for the finite number of stars on the CMD) and systematic uncertainties (which are a proxy for uncertainties in the physics of the underlying stellar models) as described in Weisz et al. ( 2014 ). Finally, as detailed in Section 3.6 and Figure 6 of Weisz et al. ( 2014 ), SFHs measured from CMDs that include the HB but do not include the oldest MSTO have larger systematic uncertainties, but are consistent with SFHs measured from the oldest MSTO-depth CMDs. Figure 1 illustrates the CMD modeling process. Panel (a) shows the observed Hess diagram of And XXI, and panel (b) shows the best-fit model for the BaSTI stellar library. A visual comparison of the model and data indicates good overall agreement. Panel (c) is a residual significance diagram, i.e., (model–data)/model, which quantifies the level of (dis)agreement. The majority of populated pixels are consistent within ∼1 σ , while only a handful of pixels are highly discrepant (i.e., 3 σ ). There are no signs of poorly fit regions of the CMD (e.g., large swaths of only black or white pixels), which indicates that the model is a good fit to the data. Panel (d) shows the cumulative SFH of And XXI, i.e., the fraction of total stellar mass formed prior to a given epoch. The solid black line is the best-fit BaSTI solution, and the gray-shaded band reflects the 68% confidence interval of the total (i.e., random plus systematic) uncertainties. The random uncertainties are negligibly small compared to the systematic uncertainties. The thin colored lines in panel (d) are the best-fit SFHs from the other four stellar libraries. These SFHs are similar in shape to the BaSTI solution, though particular features can be shifted by up to ∼2 Gyr, which is due to differences in the underlying stellar physics (e.g., Gallart et al. 2005 ). As intended, the scatter in the SFHs from different models is well captured by the gray-shaded error envelope. The small amount of apparent star formation ∼1–3 Gyr ago is likely due to a handful of blue stragglers that occupy similar portions of the CMD as