Methodology for Indiana State and County Population Projections

July 2024

Prepared by Matt Kinghorn, Senior Demographer
Indiana Business Research Center, Kelley School of Business, Indiana University

Indiana Statewide Population Projections, 2025 to 2060

Due to the unique population dynamics created by the COVID-19 pandemic, the IBRC’s 2025 projections for Indiana are based on short-term projections of the state’s components of population change (i.e., births, deaths and net migration). The projections for Indiana’s population from 2030 to 2060 were produced using the cohort-component method. U.S. Census Bureau’s population estimates for 2020 (Vintage 2022) by age and sex for Indiana and its counties serve as the base population for these projections. These projections follow the standard format of five-year age cohorts carried over five-year projection cycles. The five-year cohorts begin with the 0-4 age group and extend through 80-84, with the final age group including all individuals aged 85 and above.

2025 projections

The IBRC gathered data on Indiana’s births and deaths between 2020 and 2023 from the National Center for Health Statistics, along with annual estimates of Indiana’s net migration estimates from the U.S. Census Bureau for the same period. The IBRC then projected the values for each of these components of change for the remainder of the 2020 to 2025 period to generate a projected total population change for Indiana over this first projection cycle. This projected population change was then added to Indiana’s 2020 base population to arrive at the state’s total projected population for 2025.

The IBRC then projected Indiana’s 2025 population by age group and sex using the Hamilton-Perry method based on the U.S. Census Bureau’s population estimates for Indiana’s total population in 2023 and the state’s population by single year-of-age from 2020 to 2022. The Hamilton-Perry method is a simplified version of the cohort-component method that requires only age-sex distributions for a geographic area’s population at two different points in time. The 2025 projections by age group and sex were then controlled to Indiana’s total population projection for 2025. This approach ensured that the IBRC’s population projections for 2025 reflected the unique mortality and migration trends that were in effect during the early years of this projection cycle.

2030 to 2060 projections   

The IBRC used the cohort-component method to produce Indiana’s projections from 2030 to 2060, with the population projection for 2025 serving as the population base. The cohort-component method projects individual age-sex cohorts forward while applying specific mortality and migration rates. In addition, fertility rates are applied to the appropriate female cohorts to generate the number of births during each projection period.

Fertility: Total fertility rates (TFR) and age-specific fertility rates (ASFR) for the age groups 10-14 through 45-49 were calculated for Indiana using data from the National Vital Statistics System (NVSS) available through the CDC WONDER database. The IBRC calculated three sets of fertility rates for Indiana: 2018 to 2022 annual average, 2015 to 2022 annual average, and 2013 to 2022 annual average. The 2018 to 2022 rates were applied to the first two projection cycles (i.e., 2030 and 2035), the 2015 to 2022 rates were applied to the next two cycles, and the 2013 to 2022 rates were applied to the final three cycles.

Given that Indiana’s TFR has been in steady decline over the last 15 years, this approach of applying average rates calculated over longer time periods is a de facto assumption that Indiana’s TFR will see a very slight rebound over the next 35 years. However, the IBRC assumed that Indiana’s ASFRs will continue to change at rates like those seen over the past 15 years (e.g., declining ASFRS for younger age groups and increasing ASFRs for some older groups).

Mortality: Survival rates for each age-sex group were calculated by constructing abridged life tables with five-year age groups except for the 0-1, 1-4, and 85+ population segments. Annual life tables were calculated for each year between 2000 and 2019 using vital statistics data and population estimates from NVSS.

For age groups that showed improved mortality rates between 2010 and 2019 (i.e., the 0 to 4 groups and most 65+ groups), we assumed that rates would continue to improve at a similar rate over the projection period. For most working-age adult groups, which have had rising mortality rates in Indiana over the last 10 to 20 years, we followed a similar approach to that taken with fertility rates. For the 2030 projection, for instance, we applied average annual survival rates calculated over the 2015 to 2019 period. For the 2035 projection, we used average annual rates covering 2010 to 2019. Average annual rates from 2005 to 2019 were used for the 2040 projection, and then held constant for all subsequent projection cycles. As with fertility rates, this approach has the effect of assuming there will be slight improvements in mortality rates for these age groups. For the 5 to 14 age groups, average annual survival rates from 2015 to 2019 were held constant over the projection period.

Migration: Age and sex-specific migration rate estimates were calculated using a residual method based on survival probabilities. The first step in this approach is to construct life tables with recent data and calculate age-specific 10-year survival rates. These survival rates were then applied to the Census 2010 population counts in the appropriate age-sex groups to estimate an expected population for that same cohort in 2020. These expected population numbers approximate what the 2020 population would be in a given cohort if there was no migration over the 10-year period and deaths were the only source of population change. The difference between the expected population and the actual 2020 Census count becomes the net migration estimate.

This process is commonly referred to as the forward survival rate method. There is a similar procedure called the reverse survival rate method. Under this approach, the 2020 population counts are divided by the 10-year survival rates to calculate an expected population in 2010. Again, the difference between the expected and actual population figures for a given cohort is the net migration estimate. These two methods yield similar results, but the differences tend to widen for older age groups. For that reason, the migration rates used in these projections are an average of the forward and reverse survival rate methods. Migration rates were held constant through the projection period.

County Population Projections, 2025 to 2050

2025 projections

The 2023 county population estimates from the U.S. Census Bureau served as the base population for the 2025 projections. Total county populations were projected from 2023 to 2025 using a weighted average annual “shift-share” approach. Shift-share projections simply measure the changes in a county’s share of Indiana’s total population over time and extends that rate of change over a projection cycle. For the 2025 projections, the IBRC calculated average annual county shift-share rates between 2021 and 2023, with the rates for the most recent year given extra weight. A modified Hamilton-Perry method was used to distribute these total county population projections across age-sex cohorts. This approach ensures that these projections incorporate the impacts that the pandemic and shifting migration patterns between 2020 and 2023 had on the size and age structure of county populations.  

2030 to 2050 projections  

The IBRC used a mixed-method approach to project total county populations from 2030 to 2050. Under this approach, the IBRC generated 13 different total county population projections for each cycle using six different projection methods utilizing a variety of time periods.¹ The six projection methods were:

• Linear change: Population will change over a projection cycle by the same average annual amount as during the reference period. Three different linear projections were produced using different reference periods: 2020 to 2025, 2015 to 2025, and 2005 to 2025.
• Exponential change: Population will change over a projection cycle at the same average annual rates as during the reference period. Two different exponential projections were produced using different reference periods: 2020 to 2025 and 2010 to 2025.
• Share-of-growth: A county’s share of Indiana’s population growth over the projection cycle is the same as during the reference period. Three different share-of-growth projections were produced using different reference periods: 2020 to 2025, 2015 to 2025, and 2005 to 2025.
• Shift-share: A county’s share of Indiana’s total population will change at the same average annual rate over the projection cycle as it did during the reference period. Two different shift-share projections were produced using different reference periods: 2020 to 2025 and 2010 to 2025.
• Constant-share: A county’s share of Indiana’s total population in 2023 will remain constant over the projection period.
• Hamilton-Perry: A simplified version of the cohort-component method that only requires age-sex distributions for a geographic area’s population at two different points in time. Two different Hamilton-Perry projections were produced using different reference periods: 2015 to 2025 and 2010 to 2020.

For each projection cycle, a county’s two-highest values and two-lowest values were removed, then the remaining nine projections were averaged together. A Hamilton-Perry method based on the averages from the 2010 to 2020 and 2015 to 2025 reference periods was used to distribute total county population projections across age-sex groups. County projections were then controlled to the Indiana projections for that cycle.

Notes