  The number of successes 'n' and the normalized measure of space-time alarm
'tau' are commonly used to characterize the strength of an earthquake
prediction method and the significance of prediction results. To evaluate
better the forecaster's skill, it has been recently suggested to use a new
characteristic, the gambling score R, which incorporates the difficulty of
guessing each target event by using different weights for different alarms. We
expand the class of R-characteristics and apply these to the analysis of
results of the M8 prediction algorithm. We show that the level of significance
'alfa' strongly depends (1) on the choice of weighting alarm parameters, (2) on
the partitioning of the entire alarm volume into component parts, and (3) on
the accuracy of the spatial rate of target events, m(dg). These tools are at
the disposal of the researcher and can affect the significance estimate in
either direction. All the R-statistics discussed here corroborate that the
prediction of 8.0<=M<8.5 events by the M8 method is nontrivial. However,
conclusions based on traditional characteristics (n,tau) are more reliable
owing to two circumstances: 'tau' is stable since it is based on relative
values of m(.), and the 'n' statistic enables constructing an upper estimate of
'alfa' taking into account the uncertainty of m(.).
