There is a great deal of support for PMT in the research literature, but several limitations of the research have been noted. A common concern with implementing evidence-based treatments in community (as opposed to research) contexts is that the robust effects found in clinical trials may not generalize to complex community populations and settings. To address this concern, a meta-analysis of PMT studies coded across "real-world" criteria found no significant differences in the effectiveness of PMT when it was delivered to clinic versus study-referred populations, in routine service versus research settings, or by non-specialist versus specialist therapists (such as those with direct links to the program developers). Increased attention to the impact of cultural diversity on PMT outcomes – especially given that parenting practices are deeply rooted in culture – was called for in the 1990s;: 224   a 2013 review said the emphasis on ethnic and cultural differences was unjustified in terms of efficacy.Other limitations of the existing research is that studies tend to focus on statistically significant rather than clinically significant change (for example, whether the child's daily functioning actually improves);  there is no data on long-term sustainability of treatment effects; and little is known about the processes or mechanisms through which PMT improves outcomes.: 223 Training programs other than PMT may be better indicated for "parents with significant psychopathology (such as anger management problems, ADHD, depression, substance abuse), limited cognitive capacity, or those