Table 3: Sensitivity of line equivalent widths to age (1-13 Gyr) and metallicity ( ${\rm [Fe/H]}=-1.7$ to 0.4). $\Delta _{\rm [Fe/H]}$ represents the fractional change in [Fe/H] needed to produce a 10% variation of the index value at the zero point ($Z_\odot $, 12 Gyr). $\Delta _{\rm age}$ is the similar quantity computed with the age. $\frac{\Delta_{\rm age}}{\Delta_{\rm [Fe/H]}}$ is the ratio of fractional change in age to the fractional change in [Fe/H] required to produce the same change in the index value (Jones & Worthey 1995; Worthey 1994). High absolute values correspond to high sensitivity to Z. This quantity is evaluated around the ($Z_\odot $, 12 Gyr) zero point. The last column reproduces the values in Table 6 of Worthey (1994) and in Table 2 of Jones & Worthey (1995).
Index $\Delta _{\rm [Fe/H]}$ $\Delta _{\rm age}$ $\frac{\Delta_{\rm age}}{\Delta_{\rm [Fe/H]}}$ $\left( \frac{\Delta_{\rm age}}{\Delta_{\rm [Fe/H]}} \right)_{{\rm\tiny {W,J}}}$
         
Fe5335 0.41 0.75 1.8 2.8
Fe5270 0.42 0.74 1.8 2.3
H$\gamma $_Z 0.73 1.03 1.4  
Mg$_{\rm b}$ 0.55 0.56 1.0 1.7
H $\delta _{\rm A}$ 0.16 0.14 0.9 1.1
H $\gamma _{\rm A}$ 0.36 0.31 0.9 1.0
H$\gamma_F$ 0.23 0.15 0.7  
H$\gamma $_Vaz -3.04 -0.42 0.1  
H$\beta $ -1.62 -0.26 0.2 0.6
H$\gamma $_HR 6.50 -1.06 -0.2 0.0
H$\gamma $_VHR 11.05 -1.06 -0.1  


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