“…In one study, matching is conducted on three covariates only, in others, propensity score model specifications are not shown (n = 2), and in yet others, they are shown but not motivated (n = 3). Sometimes, the matching successfully reduces differences between the treatment and control groups (Rodríguez-Planas and Benus, 2010; Elert et al, 2015), sometimes moderate differences remain (Meager et al, 2003;Fajnzylber et al, 2009), and sometimes the imbalance reduction is not shown, impeding assessment of the quality (Rodríguez-Planas, 2010; Coad et al, 2016). One reason for the unsuccessful balancing of treatment and control groups may lie in the way that propensity scores are modeled.…”