http://voices.washingtonpost.com/answer-sheet/charter-schools/question-raised-about-new-kipp.html
A key finding of the study is that attrition at KIPP schools is not much different from attrition at comparable conventional public schools. This finding is important because past research about KIPP suggests that selective attrition -- struggling students disproportionately leaving, with more successful students staying and then scoring well on tests -- may give KIPP a substantial boost.
However, an initial analysis of the report by Professor Gary Miron of Western Michigan University concludes that this initial study report misrepresents the attrition data.
According to Miron, "While it may be true that attrition rates for KIPP schools and surrounding districts are similar, there is a big difference: KIPP does not generally fill empty places with the weaker students who are moving from school to school. Traditional public schools must receive all students who wish to attend, so the lower-performing students leaving KIPP schools receive a place in those schools."
In contrast, Miron explains, "The lower performing, transient students coming from traditional public schools are not given a place in KIPP, since those schools generally only take students in during the initial intake grade, whether this be 5th or 6th grade."
The KIPP study’s description of attrition only considers half the equation, when comparing KIPP schools to matched traditional public schools. The researchers looked at the attrition rates, which they found to be similar - in the sense of the number of students departing from schools. But they never considered the receiving or intake rate. Even though the researchers agree that the students who are mobile are lower performing, they do not take into account the reality that KIPP schools do not generally receive these students.
Much more at the link.