degree to which metallicity must predominate over age effects as follows. At $M_I$ = -1.6, the width of the red giant branch is FWHM() $\approx$ 0.6 mag, giving $\sigma$() = 0.26 mag. The photometric uncertainty at this magnitude is $\sigma$() = 0.14 mag, yielding an intrinsic RGB width of $\sigma$() = 0.22 mag, or FWHM() = 0.51 mag. Now from Figure \[fig:cmtheory\] we see that FWHM() for $\Delta Z = 1.2$ dex is $\approx 0.5$ mag, giving a change in color with respect to metallicity of 0.42 mag/dex. From the scaling rule above, the age dependence therefore goes as 0.42 / 1.7 = 0.25 mag/dex. Assuming that the observed width must be the quadrature sum of both effects, we define

$$\Delta_t \equiv {\rm FWHM ~in} ~\log \tau$$

and

$$\Delta_Z \equiv {\rm FWHM ~in} ~\log Z$$

Thus we have

$$[0.42 \Delta_Z]^2 + [0.24 \Delta_\tau]^2 = 0.51^2.$$

Setting a maximum allowable age range of $\log \tau =$1.0 dex (1.5 to 15 Gyr) and solving, we find a minimum allowable $\Delta_Z =
1.1$ dex. The data clearly mandate a large range in metallicity irrespective of the range in stellar ages which may be present.

The CM diagram implies that the distribution of metallicity 