 distributions into
                    account
    'MAD'         : mean absolute deviation in degrees for angles specified
                    in radians
    'RMS'         : root-mean-square error
    'tlin'        : t-linear statistic to measure goodness of fit for
                    circular data with angles specified in radians 
                    using fisher's correlation coefficient.
    'R2'          : squared correlation; explained variance in the linear regression case
    'expvar'      : explic