Change like this happens at the margins. Families already moving will choose one district over another due to factors; people faced with the possibility of movi

Change like this happens at the margins. Families already moving will choose one district over another due to factors; people faced with the possibility of moving (job offer) will take it or decline due to factors. Over time, these effects accumulate into something statistically observable. The caricature of people immediately putting their house on the market when a new neighbor arrives need not ever actually occur.