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This paper provides a detailed comparison of the differences in parametersderived for a star cluster from its color-magnitude diagrams depending on thefilters and models used. We examine the consistency and reliability of fittingthree widely-used stellar evolution models to fifteen combinations of opticaland near-IR photometry for the old open cluster NGC 188. The optical filterresponse curves match those of the theoretical systems and are thus not thesource of fit inconsistencies. NGC 188 is ideally suited to the present studythanks to a wide variety of high-quality photometry and available propermotions and radial velocities which enable us to remove non-cluster members andmany binaries. Our Bayesian fitting technique yields inferred values of age,metallicity, distance modulus, and absorption as a function of the photometricband combinations and stellar models. We show that the historically-favoredthree band combinations of UBV and VRI can be meaningfully inconsistent witheach other and with longer baseline datasets such as UBVRIJHKs. Differencesamong model sets can also be substantial. For instance, fitting Yi et al.(2001) and Dotter et al. (2008) models to UBVRIJHKs photometry for NGC 188yields the following cluster parameters: age={5.78+ 0.03, 6.45+-0.04} Gyr,[Fe/H]={+0.125+-0.003, -0.077+-0.003} dex, m-M={11.441+-0.007, 11.525+-0.005}mag, and Av={0.162+-0.003, 0.236+-0.003} mag, respectively. Within the formalfitting errors, these two fits are substantially and statistically different.Such differences amongst fits using different filters and models are acautionary tale regarding our current ability to fit star clustercolor-magnitude diagrams. Additional modeling of this kind, with more modelsand star clusters, and future GAIA parallaxes are critical for isolating andquantifying the most relevant uncertainties in stellar evolutionary models.

Bayesian Investigation of Isochrone Consistency Using the Old Open Cluster NGC 188