One of the (many) factors that might help explain -- or at least be associated with -- the wide variation in charter schools’ test-based impacts is market share. That is, the proportion of students that charters serve in a given state or district. There are a few reasons why market share might matter.
For example, charter schools compete for limited resources, including private donations and labor (teachers), and fewer competitors means more resources. In addition, there are a handful of models that seem to get fairly consistent results no matter where they operate, and authorizers who are selective and only allow “proven” operators to open up shop might increase quality (at the expense of quantity). There may be a benefit to very slow, selective expansion (and smaller market share is a symptom of that deliberate approach).
One way to get a sense of whether market share might matter is simply to check the association between measured charter performance and coverage. It might therefore be interesting, albeit exceedingly simple, to use the recently-released CREDO analysis, which provides state-level estimates based on a common analytical approach (though different tests, etc.), for this purpose.