  As the rate of incarceration in the United States continues to grow, a large
body of research has been primarily focused on understanding the determinants
and drivers of federal and state prison growth. However, local jail systems,
with 11 million admissions each year, have generated less research attention
even though they have a far broader impact on communities. Preliminary time
trend analysis conducted by the Vera Institute of Justice (Vera) uncovered
disparities in county jail incarceration rates by geography. Contrary to
assumptions that incarceration is an urban phenomenon, Vera discovered that
during the past few decades, pretrial jail rates have declined in many urban
areas whereas rates have grown or remained flat in rural counties. In an effort
to uncover the factors contributing to continued jail growth in rural areas,
Vera joined forces with Two Sigma's Data Clinic, a volunteer-based program that
leverages employees' data science expertise. Using county jail data from 2000 -
2013 and county-specific demographic, political, socioeconomic, jail and prison
population variables, a generalized estimating equations (GEE) model was
specified to account for correlations within counties over time. The results
revealed that county-level poverty, police expenditures, and spillover effects
from other county and state authorities are all significant predictors of local
jail rates. In addition, geographic investigation of model residuals revealed
clusters of counties where observed rates were much higher (and much lower)
than expected conditioned upon county variables.
