PMT is one of the most extensively studied treatments for childhood disruptive behaviors. PMT tended to have larger effects for younger children than older children, although the differences between age groups were not statistically significant. Improvement in parental mental health (depression, stress, irritability, anxiety, and sense of confidence) as well as parental behavior is noted.  Improvements in child and parent behavior were maintained up to one year after PMT, although the effects were small; very few studies have been done on the durability of the effects of PMT.Families from economically disadvantaged backgrounds were less likely to benefit from PMT than their more advantaged counterparts, but this difference was attenuated if the low-income families received individual rather than group treatment. Overall, group formats of PMT delivery were less effective than individual formats, and the addition of individual therapy for the child did not improve outcomes.  Parental psychopathology, substance abuse, and maternal depression are associated with less successful outcomes; this may be because the "parents' ability to learn and consolidate the skills being taught" is affected, or parents may not be able to stay engaged in the program or translate the skills acquired to the home.Furlong et al (2013) concluded that group-based PMT is cost-effective in reducing conduct problems, and improving parental health and parenting skills, but that there is not enough evidence that it is effective on the measures of "child emotional problems and educational and cognitive abilities".