↑ Lightbown 1990 cited in Ellis 1994. ↑ Doughty & Williams 1998. ↑ Ellis 2002. ↑ Lightbown & Spada 1990. ↑ Lyster & Ranta 1997. ↑ Lyster & Mori 2006. ↑ Tarone, Bigelow & Hansen 2009. ↑ Allwright & Hanks 2009. ↑ Tarone & Swierzbin 2009. ↑ Horwitz 1986. ↑ Horwitz 1986, pp. 685–686. ↑ Pfeffer 1964. ↑ Shulman, L.S. (1987).