The oral hygiene Skill Achievement Index (S.A.I.) is a general method for the direct, quantitative evaluation of oral hygiene skill. The S.A.I. evaluates a person's ability to manipulate a toothbrush and floss in a given fashion. It is applicable to any brushing or flossing procedure. The Index was developed to fulfill all the postulated requirements of a perfect measuring instrument. The key feature of the Index is the evaluation of the position and the motion of the cleaning device on each tooth surface. Consequently, the Index provides a format for the instruction and evaluation of oral hygiene skill. Furthermore, if used in conjunction with other dental indices, it enables one to evaluate hygiene procedures and devices in a controlled fashion.