The linguistic (Chall 1948; Fang 1966; Kiyokawa 1990; Messerklinger 2006;<cite> Kotani et al. 2014</cite>; Kotani & Yoshimi 2016; Yoon et al. 2016) , and learner features<cite> (Kotani et al. 2014</cite>; Kotani & Yoshimi 2016) used in this study were originally described elsewhere.