[nouveaux]

This seems like an important point:

"The differences between the experiments could also be behind the difference in size of correlation. Each study used different measures of achievement. The marshmallow test in the replication had a maximum wait of only 7 minutes, compared to 15 to 20 minutes in the original (although most of the predictive power in the new study was found to be in the first 20 seconds). The replication created categories for time waited (e.g. 0 to 20 seconds, 20 seconds to 2 minutes, and so on), rather than using time as a continuous variable. It also focused on children with parents who did not have a college education – too many of the children with college-educated parents waited the full seven minutes. The original study drew its sample from the Stanford community."