A tale of two variances: between and within.

In response to Worobey and Brazelton's thoughtful and welcome commentary on my assessment of the effectiveness of a Brazelton-based newborn intervention, 2 points are made. The first is that a healthy difference of opinion exists regarding the effectiveness of such interventions; I view the available evidence as less striking than they, but not as limited as they feel my initial study report implies. The second point of this essay is to provide empirical support for my critics' contention that how an intervention is delivered is as important as the fact that an intervention is delivered. By presenting data excised from my original submission to Child Development, it is shown that variation in parents' interest in, involvement with, and enjoyment of the Brazelton intervention was directly related to the intervention's outcome within the joint mother-father treatment condition--even often controlling for background factors related to engagement of the intervention. It is thus concluded that assessment of the process of intervention is critical to any complete evaluation of an intervention, regardless of the results of experimental-control group comparisons.