This paper utilises a novel method for the classification of subjects with osteoarthritic and normal knee function. The classification method comprises a number of different components. Firstly, the method exploits the Dempster-Shafer theory of evidence allowing for a degree of ignorance in the subject's classification, i.e., a level of uncertainty as to whether a gait variable indicates osteoarthritis or not. Secondly, the inclusion of simplex plots allows both the classification of a subject, and the contribution of each associated gait variable to that classification, to be represented visually. As a result, the method is further able to highlight periodic changes in a subject's knee function due to total knee replacement surgery and subsequent recovery. The visual representation enables a simple clinical interpretation of the results from the quantitative analysis.