Pornchanok Kittiprasert. (1998) An error analysis of business letters written by vocational students. National Institute of Development Administration,:ม.ป.ท.Pornchanok Kittiprasert. 1998. An error analysis of business letters written by vocational students. ม.ป.ท.:National Institute of Development Administration,;Pornchanok Kittiprasert. An error analysis of business letters written by vocational students. ม.ป.ท.:National Institute of Development Administration, 1998. Print.