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Tests the null hypothesis that two samples of recoveries belong to the same bivariate distribution.',

References:

K. V. Mardia. 1967. A Non-Parametric Test for the Bivariate Two-Sample Location Problem. Journal of the Royal Statistical Society. Series B (Methodological) 29(2): 320-342. abstract

Batschelet, E. 1972. Recent Statistical Methods for Orientation Data. Pages 61-91, In Animal Orientation and Navigation, NASA SP-262. pdf

The procedure for the Ties Averaged case (Method 4) is described in 

Robson, D.S. 1968. The Effect of Ties on Critical Values of Some Two-Sample Rank Tests. Biometrics Unit Technical Reports, Cornell Univ. BU-258-M. pdf