A considerable amount of the controversy about the Dodo bird verdict relates to the meta-analytic methods used in research. These generate a lack of clear psychotherapeutic evidence for the support of either side in the debate. Meta-analytic studies have compared the effect sizes of different treatments, but have not been reliable in finding a consistent effect size. This could be because of several confounding variables. For example, many researchers are said to "have an agenda" when conducting meta-analyses, cherry-picking experiments they want to use in their study in order to produce the results they want. This pre-determined skewing results in unfair representations of the research. "Agenda"-based meta-analyses are confounded with the researcher's political, social, and economical opinions. Since psychologists are given the power to choose which studies are used in a meta-analysis, personal biases are involved, and the meta-analysis will produce biased results if the researcher is not careful in controlling for his or her own opinions.