The misuse of the confidence ellipse in evaluating statokinesigram

ABSTRACTRocchi MBL, Sisti D, Ditroilo M, Calavalle A, Panebianco RThe misuse of the confidence ellipse in evaluating statokinesigram Ital J Sport Sci 2005: 12: 169-172 The aim of this brief technical note was to focus on a widely used method for evaluating the statokinesigram: the confidence ellipse. Wepointed out that several authors misinterpret the meaning of CE, which is actually well defined in statistics. Anyway we illustrated somelimitations when using confidence ellipse in evaluating statokinesigram. Finally we suggested substituting confidence ellipse with thestandard ellipse. KEY WORDS: posturography, statokinesigram, confidence ellipse, standard ellipse The problem of quantifying the oscillation of an im-mobile standing subject in order to evaluate his ba-lance ability is a major issue in posturography andstabilometry.The methods employed to assess balance behaviorvary widely among authors. The analysis of the so-called statokinesigram, i.e. the projection of the centerof pressure (CoP) on a bidimensional plane by meansof a force platform, is often used. Circles and rectan-gles have been fitted to the statokinesigrams in the x-y plane [1-3], but the most accepted method is theanalysis of the 90% or 95% confidence ellipses; wewill generically denote them as (1- α% confidence el-lipses) of the postural sway [4-7]. This parameter isoften used alone, or combined with the length of thesway to obtain the LFS (Length as Function of Surfa-ce) index [8-9]. In both cases, the area of the confi-dence ellipse is used as a measure of energetic expen-diture of the subject to maintain his balance [8-10].We think it is important to underline some basic stati-stical aspects of the confidence ellipse, to illustratesome of its limitations in evaluating statokinesigram.