I believe more than one study in Milwaukee have led researchers to this conclusion. There was another study put out by the Brookings Institute on Milwaukee charter schools in 2009 with a similar conclusion:
The Impact of Milwaukee Charter Schools on Student AchievementStéphane Lavertu and John Witte
Our analysis also indicates that whether or not a student switches schools has
a strong, negative, and statistically significant correlation with student
performance on reading and mathematics achievements tests, and that the
magnitude of this factor’s impact exceeds that of the organizational factors on
which our study focuses. Put differently, our results are consistent with the
notion that stability in a student’s life is a better predictor of academic
performance than organizational factors. One of the policy implications of this
study is that it is important to better understand and deal with instability in
school attendance in urban school districts.
The impact of charter schools on student performance on reading and
mathematics achievement tests is not the only factor one should consider when
assessing the value of charter schools. One might be more concerned about the
relationship between charter school attendance and college enrollment rates, for
example. And choice options often are associated with higher levels of parental
engagement and satisfaction, regardless of student performance on achievement
tests. However, for the many policymakers who consider achievement test
results to be of primary importance, our study indicates that charter school
attendance in Milwaukee at best has a modest positive affect on such
achievement. In other words, charter schools may contribute positively to
improving the education of urban youth, but they should not be expected to be
the silver bullet that some reformers seek.