The most interesting correlation which the authors provide are:

1. Answering problem sets faster (higher speed) significantly correlated with less studying (r=-.62) and practicing (r=-.49), but not higher final exam grades (r=.127).
2. Earliness (less procrastination) correlated with final exam grades (r=.301) but not speed (r=.14) or studying hours (r=-.138).
3. More studying correlated with more practicing (r=0.606).

These are the results:

1. Each extra hour using the practice tool was associated with a 1.04% increase in the final exam score.
2. Each extra hour using the e-book, excluding the practice tool usage, had no significant residual correlation with exam scores.
3. As expected, measures of prior preparation (GPA, Math score) were positively correlated with exam score, as was earliness.
4. The effect of speed on early problem sets was non-significant after controlling for other covariates.