Solid-line histogram denotes the compiled data of high-z type 1 quasars from the literature (McIntosh et al. 1999; Netzer et al. 2004; Marziani et al. 2009; Greene et al. 2010). Filled circles with error bars denote the data of SDSS type 1 quasars at 0 < z < 1 (Risaliti et al. 2011). The frequency distributions of the two datasets are normalized by the number of objects in the bin at the peak of their frequency distributions. The arrow denotes the EW_rest([O iii]) value of SDSS J1707+6443 measured in our work.