TY - JOUR T1 - Letter: The retrospective study JF - Canadian Medical Association Journal JO - CMAJ SP - 13b LP - 13 VL - 114 IS - 1 AU - T. W. Anderson AU - M. J. Ashley AU - A. E. Clarke Y1 - 1976/01/10 UR - http://www.cmaj.ca/content/114/1/13b.abstract ER -