has also been criticized by Lawrence Mishel, who argued that Hoxby's "assessment of school outcomes is based on the share of students who are proficient at reading or math but not the average test score of the students. That's like knowing the poverty rate but not the average income of a community—useful but incomplete." How representative the study is has also been criticized, as the study is only of students in Chicago. Learning gains studies A common approach in education evaluation research is to compare the learning gains of individual students in charter schools to their gains when they were in traditional public schools. Thus, in effect, each student acts as his/her own control to assess the impact of charter schools. A few selected examples of this work find that charter schools on average outperform the traditional public schools that supplied students, at least after the charter school had been in operation for a few years. A possible limitation of this type of study is that it does not automatically distinguish between possible benefits of how the school operates (e.g. school structure) and possible peer effects, that is, effects of students on each other. At the same time, there appears to be a wide variation in the effectiveness of individual charter schools. Meta-analyses A report issued by the National Alliance for Public Charter Schools, released in July 2005 and updated in October 2006, looked at twenty-six studies that make some attempt to look at change over time in charter school student or school performance. Twelve of these find that overall gains in charter schools were larger than other public schools; four find charter schools' gains higher in certain significant categories of schools, such as elementary schools, high schools, or schools serving at risk students; six find comparable gains in charter and traditional public schools; and, four find that charter schools' overall gains lagged behind. The study also looks at whether individual charter schools improve their performance with age (e.g. after overcoming start-up challenges). Of these, five of seven studies find that as charter schools mature, they improve. The other two find no significant differences between older and younger charter schools. A more recent synthesis of findings conducted by Vanderbilt University indicates that solid conclusions cannot be drawn from the existing studies, due to their methodological shortcomings and conflicting results, and proposes standards for future meta-analyses. National Center for Education Statistics