Harper [3]	5–8 mm
McGee [4]	8 mm
Hickey, Boucher and Woelfel [6]	8–10 mm
Martone [7]	8–10 mm
Schiffman [8] 507 subjects	–
Ehrlich (1975) 430 casts	–
Watt (1976)	8–10 mm
Ortman and Tsao [10] 38 casts	12.45 mm
Mavroskoufis and Ritchie [11] 64 students	10.2 mm
Grave and Becker [14]	12–13 mm
Lau and Clark [16] 82 subjects	9.17 mm (mid point), 12.71 mm (post border)
Sawiris [29]	8.5 mm
Ehrlich and Gazit [9] 507 casts	12.3 mm