This briefing concerns two measures of Ambient Positional Instability (API) among teachers in the state of Illinois: cohort retention and churn. The teacher population includes full time public school teachers in the base year of AY2007-2008 and who were followed longitudinally through AY2011-2012. The state, district, and school level cohort retention for elementary, middle and high school teachers over the five years are provided here, as are the cohort retention rates in the five largest school districts in Illinois. Population churn rates, which include both leavers and newcomers to the Illinois system, are reported within the state over the years covered. A rationale for the work is that high instability and regional differences in the instability rates can have serious implications for designing school level interventions, especially for those designed to be implemented over the course of multiple years, and for controlled trials on such interventions. Challenges and techniques used to mitigate problems encountered using publicly available datasets are also discussed.