α-element abundance over iron, computed by averaging [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] as a function of [Fe/H] in member stars of four calibration open clusters. The abundances from WG11 are shown in black and those from WG10 in blue. The red circles mark the median WG11 abundances within 1σ, 2σ, and 3σ. The green circles indicate the same quantities but computed with WG 10 results.