Table 3: Mean gradients of age, [M/H], and metallicity, derived with different indices for LDEGs. $\sigma $: typical deviation; N: number of galaxies; $N_{\rm eff}$: effective number of points, $N_{\rm eff}=\left [\sum (1/\sigma ^2)\right ]^2$/ $\sum \left [(1/\sigma ^2)^2\right ]$; t: t-statistic to check the hypothesis "mean $\ne $ 0''; $\sigma _{\rm exp}$: typical deviation expected from errors; $\alpha $: level of significance to reject the hypothesis " $\sigma =\sigma _{\rm exp}$''. The last column contains the t-statistic used to test the hypothesis "mean grad[M/H] = mean grad[M/H]'' (with different indices).
  Mean $\sigma $ N $N_{\rm eff}$ t $\sigma _{\rm exp}$ $\alpha $ t
grad ($\log$ age) $ 0.082\pm 0.015$ 0.117 61 19.62 3.1 0.032 1.63E-13  
grad [M/H] $-0.206\pm 0.019$ 0.151 61 17.25 5.7 0.075 4.3E-22  
grad [M/H] (CN2-H$\beta $) $-0.582\pm 0.032$ 0.249 61 23.14 11.2 0.111 2.6E-31 5.94
grad [M/H] (C4668-H$\beta $) $-0.459\pm 0.028$ 0.219 61 21.78 9.8 0.078 0.000 4.26
grad [M/H] (Fe4383-H$\beta $) $-0.197\pm 0.025$ 0.197 61 15.31 3.9 0.109 1.2E-14 0.14
grad [M/H] (Mgb-H$\beta $) $-0.407\pm 0.039$ 0.302 61 14.53 5.1 0.138 5.3E-31 2.30
grad [M/H] (Ca4227-H$\beta $) $-0.238\pm 0.034$ 0.269 61 19.18 3.4 0.178 2.9E-7 0.45

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