it computation of explained variance: 
                    (var(y) - var(y-yhat)) / var(y)
                    1 = perfect prediction, &lt;0 = worse than chance
    'identity'    : computes the proportion of trials that are identified correctly 
 
    NOTE: notation '-x' with x one of the above is also allowed. This way
    error measures such as 'MAD' can be used as a performance measure by
    dml.permutation or dml.gridsearch using '-MAD'
 
    EXAMPLE
    s = dml.statistic('accuracy',D,P)
    s = dml.st