Circular or angular data occur in many fields of applied statistics. A common problem of interest in circular data is estimating a preferred direction and its corresponding distribution. It is complicated by the wrap-around effect on the circle, which exists because there is no natural minimum or maximum. The usual statistics employed for linear data are inappropriate for directional data, as they do not account for its circular nature. The robustness of the three common choices for summarizing the preferred direction (the sample circular mean, sample circular median and a circular analog of the Hodges-Lehmann estimator) are evaluated via their influence functions.
Otieno, B. Sango and Anderson-Cook, Christine M.
"Effect Of Position Of An Outlier On The Influence Curve Of The Measures Of Preferred Direction For Circular Data,"
Journal of Modern Applied Statistical Methods: Vol. 4
, Article 9.
Available at: http://digitalcommons.wayne.edu/jmasm/vol4/iss1/9