Dependence of the unresolved binary fraction with a high mass ratio (q ≥ 0.6), f_b, with the cluster-wide parameters, i.e. log(Age) (left), distance (middle), and [Fe/H] (right) for the 42 OCs. The data points are coloured by the mean mass of MS primary stars. The grey error bars represent the Poisson uncertainties in the binary fractions and 1σ errors in the cluster parameters.