Although Dawson was able to discount screening as a potential confound due to the program being non-selective (admission based on date of application), no data was provided on pre-programming levels to indicate whether the groups differed prior to enrollment in the Montessori program. In addition, no statistical control procedures were applied to control for potential demographic confounds such as gender and economic status. Lastly, Dawson noted that parental selection “could not be ruled out” as a rival explanation.