The recent numerical study of Song and Banner (JPO, 2002, henceforth SB02) proposed a generic breaking onset threshold parameter based on the wave energy convergence rate at the maximum of an evolving two-dimensional nonlinear gravity wave group in deep water. In a companion paper, these authors found that this mechanism for breaking onset was not modified significantly by strong wind forcing and shear. SB02 also suggested that the parametric growth rate just prior to breaking onset might control the strength of breaking events. This presentation describes the results of a detailed observational study aimed at validating their proposals. For the nonlinear wave groups investigated, the observations reproduced the evolutionary trends of the SB02 model predictions in regard to key evolution times and confirmed the breaking threshold level of 0.0015 for their proposed parametric growth rate. In regard to strength of breaking, the proposed dependence of breaking strength on the level of their parametric growth rate just before breaking onset was not found to be valid. However, by using a slight modification of the SB02 parametric growth rate, it appears possible to achieve a consistent predictor for the observed breaking strength.