Notes on 2002-2020 data and methodology
The county-level and statewide data for the number of registered voters, total votes, election day votes, and absentee/early votes comes from the Indiana Election Division. In some cases the number of absentee votes plus election day votes is not equal to the total number of votes, and so this graph assumes the total votes and absentee votes numbers are correct and derives the election day votes by simply subtracting absentee votes from total votes. While the raw numbers might not be entirely accurate, they are "accurate enough" in the sense that none is so far off as to skew the overall picture of turnout in a given year, or trends across multiple years.
The statewide Voting Age Population (VAP) and Voting Eligible Popualtion (VEP) numbers come from the United States Election Project run by Michael P. McDonald at the University of Florida. It's important to note that both of these numbers are just estimates, with the VAP numbers coming directly from U.S. Census Bureau projections, and VEP representing an adjustment to VAP to account for non-citizens and felons. The Election Project also uses a simple linear interpolation to adjust both figures from the month of the VAP calculation to the month of the Primary or General election. Because the Election Project doesn't publish these numbers for midterm primaries, I have applied the exact same methodology to fill in those gaps in this data. More information on how VEP is calcualted can be found here.
I have applied a similar methodology to estimate county-level VAP and VEP data. Using U.S. Census Burea figures for county populations in each decennial census, I calculated the population share for each county at the start of each decade. I then applied a linear interpolation to determine the population share every two years, and multiplied the statewide VAP and VEP estimates for each election by the relevant county/year population share figure. Again, the numbers may not be entirely accurate, but the estimate should be "accurate enough" to show both an overall picture and trends for a given county.