How is the ineligible felon population estimated?
The number of ineligible felons depends on state law. Some state permanently disfranchise felons and a few let even prisoners vote. Statistics drawn from various Department of Justice report the prison, probation, and parole population of the United States. Links to these reports are provided on individual election voter turnout data web pages. These statistics are matched against the various state felony disfranchisement laws compiled by the Sentencing Project to estimate the number of ineligible felons in a given state.
The Department of Justice Bureau, Office of Justice Statistics releases statistics of the correctional population from Jan. 1 through Dec. 31 for a given year. I use the most approximate Jan. 1 values, where available, since these include revised Dec. 31 statistics from the previous year. All Prisoners and Parolees are assumed to be felons, and half of Probationers are felons. The rational of estimating half of probationers as felons is drawn from DOJ reports; starting in 2011, DOJ reports directly the number of persons on probation who are felons and no estimation is required. A blank or zero indicates the category of felons is allowed to vote in that state. For the United States totals, I include persons in the Federal corrections system.
This can be a crude methodology, as some states disenfranchise only persons who commit specific crimes. Unfortunately, the Department of Justice reports do not break down persons in the correctional population to this fine degree.
A word on permanently disfranchised felons
The number of permanently disfranchised felons in the states that have some form of post-correctional voting restriction is not computed, since time-series statistics on recidivism, deaths, and migration of felons are largely unavailable. Chris Uggen and Jeff Manza estimated the number of permanently disfranchised felons in the 2004 election. Because their methodology has not been updated for a longer time period, I have not adopted their adjustment.