Tennessee population projections developed by the Boyd Center for Business and Economic Research at the University of Tennessee provide an important look into the state’s future. They give us a glimpse at the population and demographic makeup of the state’s 95 counties through 2070. The projections are built using data about the state’s current population— including age, race and Hispanic ethnicity—that is supplemented with information about births, deaths and migration into and out of the state.
But what about the future population of the state’s 345 cities and towns? This is one of the questions most often asked at the State Data Center and it’s a somewhat more difficult question to answer. This is because the Boyd Center’s population projection inputs are based on county-level data. Those inputs are used to produce annual projections with a similar geographic basis.
In response to these requests, we’ve developed two simple approaches that researchers, planners and analysts can use to project the total population of municipalities and unincorporated county balance. These methods don’t support the analysis of future age, sex and race/ethnicity characteristics, but they serve the general planning needs of many users.
Two datasets are needed for this analysis:
- Boyd Center Population Projections 2018-2070
- Subcounty Resident Population Estimates, City and Town Population Totals: 2010-2020, US Census Bureau
The Boyd Center Projections provide county-level projections for each year from 2019-2070 using a 2018 base year. The Census Bureau population estimates provide 10 years of population estimates for each city and town from 2010 to 2020, which are used to determine the trend of population change.
For our sample analysis, we’ll be using the Town of Pegram, Tennessee, in Cheatham County to demonstrate the options. Cheatham County has a population of 41,100 people and is located immediately west of Nashville/Davidson County. Pegram (pop. 2,056) is one of four incorporated areas in the county that also includes Ashland City, Kingston Springs and Pleasant View. About 65 percent of the county’s residents live in the unincorporated areas.
Subcounty Projection Options
We have developed two methods for projecting the future population of Tennessee cities and towns that are designed to be simple and can be performed using Microsoft Excel. A sample Excel workbook outlining the procedures is available for download.
The length of projection desired is the primary consideration when determining which method to use. An option for a short-term projection of five years or less as well as a longer-term option are included below and in the sample Excel workbook.
Regardless of the length of the projection horizon, recent population changes in the incorporated and unincorporated parts of a county are used. Both approaches incorporate 2010 to 2020 subcounty population data from the U.S. Census Bureau’s Population Estimate Program.
Because the methods rely on historic subcounty data to project future sub-county population, they incorporate some basic assumptions:
- Changes to housing stock, employment base and infrastructure do not dramatically alter an area’s desirability or accessibility
- Desirability or accessibility of other places does not change due to housing, employment or infrastructure changes
- Changes or lack of changes to corporate limits have been regular and have not induced sudden and significant population changes that are reflected in the trendline
- Rates of subcounty components of population change (births, deaths and migration) in the previous years and the projection years remain consistent
More sophisticated models can incorporate changes to these assumptions at the subcounty-level. However, the universal availability of historic population data for the state make the acceptance of these somewhat crude assumptions worthwhile. Regardless of the method used, all projections should be revisited when new input data are available. Because the Census Bureau’s population estimate series is revised regularly, annual updates to municipal population projections are feasible, even in years when new Boyd Center projections are not.
Download the Sample Excel Workbook
This workbook provides an overview of the steps to create a both short and long-term projections for a Tennessee municipality based on the 2018 Boyd Center Population Projections.
Short Term: Linear Population Trend of a Single Municipality
The first option for projecting the future population of a city or a town doesn’t actually incorporate the Boyd Center projections. The process uses recent population estimates to extrapolate the future population on a linear trendline. This option relies on the assumption that policies and preferences in the preceding period will remain consistent into the projection period.
For the sample community, a projection of the linear trendline to 2025 shows a slight downward trajectory. Because recent population has only fluctuated by 55 people during the ten-year period, the trend appears reasonable. However, statistically the trend does not appear very reliable (R-squared = 0.0154) and additional years of historic population may be needed to strengthen the analysis for this community.
- Short-term projection of total population for a single municipality limited to three to five years
- Determination of direction of population change in a county or subcounty geography (increase v. decrease)
- Trend projections for a single municipality are not constrained by the county’s total projected population and may reflect growth that is too optimistic or pessimistic than the projected growth of the county as a whole
- In some cases, linear trendlines produce low reliability when measured using r-squared (regression coefficient). In the case of population change over time, R-squared calculates the proportion of population change attributable to an additional year in the series. If r-squared is low (perhaps than less than 0.3), evaluate the resulting trendline for its reasonableness, incorporate additional years of population history and consider employing the long-term solution to validate the direction and magnitude of change.
Long Term: Disaggregate County-Level Projection Using 2010 to 2020 Trend of the Subcounty Proportions of County Total Population
Disaggregation is the process of separating data into its component parts. For this analysis, we’ll be disaggregating the county-level population projections to each city, town and balance of the county. The process ensures that the sum of the population of all municipalities and the county’s unincorporated balance equal the Boyd Center Projection in future years.
The long-term disaggregation approach calculates the proportion of the population living in each subcounty part as a share of the county’s total population. However, rather than relying upon a single point in time to allocate future population, a linear trendline of subcounty share of total population calculated for each component geography as is shown in Figure 2.
This trendline incorporates different rates of population change that occur across the county and uses this momentum when disaggregating the county-level projection by assigning increasing proportions to faster growing communities and diminishing shares to slower growing ones.
In our sample community, population estimates show that between 2010 and 2020, the percentage of the county’s total population living in Pegram fell from 5.3 percent to 5 percent (Figure 3). A trendline extrapolating that trend shows it could fall to 4.5 percent by 2040. The R-squared of this trendline is 0.832, and for Pegram’s population time-series this approach is a more accurate predictor of annual change.
- Projections of subcounty population for a single municipality or all municipalities in a county one or more years into the future
- Although the Boyd Center data includes age, sex and race/ethnicity, only total population can be projected forward for a subcounty area
- For incorporated areas which cross a county line, the procedures for the long-term solution must be replicated for each county that the city or town intersects.
For more information about developing subcounty population projections for Tennessee cities and town using the Boyd Center Population Projections, contact the State Data Center at (865) 974-6070 or email@example.com.