Using Synthetic Adjustments and Controlling to Improve County Population Forecasts from the Hamilton–Perry Method

Tayman and Swanson (J Popul Res 34(3):209–231, 2017) found in Washington State counties that a forecast based on the Hamilton–Perry method using a synthetic adjustment (SYN) of cohort change ratios and child-woman ratios had greater accuracy and less bias compared to forecasts holding these ratios constant (CONST). In this paper, we assess the robustness of SYN’s efficacy by evaluating forecast accuracy, bias, and distributional error across age groups in counties nationwide. We also investigate whether forecast errors and their patterns change for SYN and CONST if forecasts by age and gender are adjusted to an independent total population forecast for each county. Our main findings are as follows: (1) SYN lowers forecast error compared to CONST whether the forecasts are controlled or not; (2) controlling also leads to the improvements in forecast error, often exceeding those in SYN; and (3) using SYN and controlling together has the greatest effect in reducing forecast error. These findings remain after controlling for population size and growth rate, but the positive impacts on forecast error of SYN and controlling are most evident in counties with less than 30,000 population and that grow by 15% or more.


Introduction
Although the general idea of a cohort change ratio (CCR) has been around for at least 100 years (Hardy and Wyatt 1911) and it has been widely used to generate population forecasts since their "re-introduction" by Hamilton and Perry (1962), CCRs have largely remained a tool of applied demographers who generate population forecasts (Smith et al. 2013, pp. 176-179). The Hamilton-Perry (H-P) method is a short-cut variant of the cohort-component method (CCM) that has much smaller data requirements than its more data-intensive cousin, while still providing forecasts of the population by age and gender (as well race, ethnicity, or other characteristics, if so desired). Instead of specific rates for each component of population change, as used in the CCM, H-P method forecasts for all but the youngest age groups are based on CCRs that combine the effects of mortality and migration. Child-woman ratios (CWR) are used to forecast the youngest age groups. CCRs and CWRs are most often obtained from the two most recent censuses, but they can be based on age distributions from any two points in time.
Consequently, the H-P method requires much less time and resources to implement than the CCM. Not surprisingly, it has mainly been used for subcounty geographic areas where fertility, mortality, and migration data are non-existent, unreliable, or difficult to obtain (Baker et al. 2014;Smith et al. 2013, p. 176;Swanson et al. 2010). This method has also gained acceptance as research has demonstrated its practical value and reasonable error levels in forecasting populations (Baker et al. 2017, Chapter 4;Kodiko 2014;Smith and Shahidullah 1995;Smith and Tayman 2003;Swanson and Tayman 2017). Although the H-P method has been primarily used for subcounty geographic areas, its minimal data requirements combined with the ability to forecast age and other characteristics make it attractive for use at higher levels of geography such as states and counties when information about the components of population change is not needed (Hauer 2019;Rayer and Wang 2020).
Assessments of the H-P method have been based on the usual assumption that the CCRs developed over the base period and CWRs developed for the launch year are held constant over the forecast horizon (horizon) (CONST). Smith et al. (2013, p. 179) discuss the possibility of relaxing this assumption by averaging CCRs and CWRs from several recent censuses (AVG), by extrapolating historical trends in these ratios (TREND), or by using a synthetic approach based on CCR and CWR forecasts from a population in a larger geographic area (SYN). AVG and TREND make individual adjustments to the CCRs and CWRs for each area being forecast. SYN, used frequently and successfully in state and local forecasting (Smith et al. 2013, p. 65;Williamson 2013), does not make area-specific adjustments, but applies the same proportionate change to each area based on a forecast for a larger geographic area (i.e., state changes applied to each county). SYN globally incorporates information from the horizon being forecast, while AVG and TREND rely solely on geographic-specific historical patterns. Tayman and Swanson (2017) evaluated these approaches for modifying CCRs and CWRs for Washington State counties and compared their forecast errors to 1 3 Using Synthetic Adjustments and Controlling to Improve County… errors from CONST. They evaluated 10-year forecasts using a 2000 launch year and a 2010 horizon year, historical data from 1980 to 2000 for each county, and state-level CCRs and CWRs from 2000 and 2010. They found that (1) forecasts based on the CONST were almost universally better (lower error) than forecasts based on TREND; (2) AVG fared much better against CONST; its forecasts, generally had less bias and greater accuracy; (3) SYN outperformed forecasts from TREND and CONST (less bias, greater accuracy, and less allocation error); and (4) SYN also outperformed AVG, but to a lesser extent compared with TREND and CONST. These findings suggested there is more to be gained in the H-P method by applying a global adjustment covering the horizon rather than basing adjustments on area-specific historical changes.
This paper extends Tayman and Swanson (2017) in two fundamental ways. First, Washington State is a high growth state and its counties lacked variation in growth rates, which are almost always positive. We assess the robustness of the efficacy of SYN by evaluating forecast accuracy, bias, and distributional error in counties nationwide. Before this study, evaluations of the H-P method usually focused on counties in a single state. Second, their study only examined uncontrolled H-P forecasts. It is advisable to control H-P forecasts by age and gender to independent forecasts of the total population (Baker et al. 2020;Smith et al. 2013, p. 181;Swanson et al. 2010). Such "controlling" is common when forecasting or estimating the populations of substate areas such as counties (Pittenger 1976, pp. 80-89;Smith et al. 2013, pp. 258-272;Swanson and Tayman 2012, pp. 254-265). We examine whether forecast errors and their patterns change for SYN and CONST by comparing uncontrolled H-P forecasts with H-P forecasts adjusted to an independent total population forecast for each county. We also offer suggestions and guidelines for implementing the H-P method in county-level forecasts.

Hamilton-Perry Method Alternatives
The H-P method uses CCRs, which are calculated by dividing the population age x in year t by the population age x-10 in year t-10. CCRs are usually calculated separately for males and females. These CCRs are applied to each age/gender group in year t to provide forecasts by age in the year t + 10. Given the nature of the CCRs, 10-14 is the youngest 5-year age group for which forecasts can be made if there are 10 years between censuses. Children younger than age 10 are forecast using CWRs from the launch year (i.e., males or females 0-4/females ages 15-44 and males or females ages 5-9/females ages 20-49). Equations 1 through 3 represent the usual application of the H-P method (i.e., holding CCRs and CWRs from the most recent 10-year period constant over the horizon): (1) n P x+10,g,t+10 = n CCR x,g,t × n P x,g,t (Ages 10+), where n is the width of the age group, x is the beginning of the age group, g is gender, t is the launch year, P is the population, CCR is the cohort change ratio, CWR is the child-woman ratio, and FP is the female population.
H-P forecasts using SYN are computed by (bold indicates the state): where SYNCCR is the age-specific CCR adjustment for the state, and SYNCWR is the age-specific CWR adjustment for the state.

Total Population Controls
An independent forecast of the 2010 total population for each county is needed to create the controlled forecast alternatives for CONST and SYN. The base period for these forecasts is 1990-2000. Controlling is accomplished by applying a countyspecific proportionate adjustment to the initial age and gender-specific forecasts so they add to the independent county total population forecast. The adjustment factor is the total population forecast obtained by summing the age and gender forecast divided by the independent total population forecast. An average of five extrapolation methods is used to construct the independent total population forecast. The first method holds the population constant at its 2000 level. The second method assumes the population from 2000 to 2010 changes at the same numeric amount as it did during the base period (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000). The third method assumes the population from 2000 to 2010 changes at the same percentage amount as it did during the base period. The fourth method assumes the change in the share of the county to the state population in the base period is the same between 2000 and 2010. The final method assumes that the county's share of the state's population change during the base period will be the same between 2000 and 2010. A detailed discussion of these methods is found in Smith et al. (2013, Chapter 8).

Forecast Error Measures
We analyze several commonly used measures that capture three dimensions of forecast error-accuracy, bias, and allocation error (Swanson 2015;Swanson et al. 2011). Error is defined as the difference between the simulated forecast and census count. The mean algebraic percent error (MALPE) measures bias in which positive and negative values offset each other. A positive MALPE reflects the tendency for the forecasts to be too high on average and a negative MALPE reflect the tendency for the forecasts to be too low on average. The mean absolute percent error (MAPE) measures forecast accuracy in which positive and negative errors do not offset each other. It shows the average percentage difference between the forecast and observed population, ignoring the sign of the error. 1 The MAPE and MALPE are based on forecast errors for a particular geographic area. Another perspective views the misallocation of the forecast across geographic space or a given variable, such as age. Our focus here is not on geographic misallocation, but on the accuracy of the age distribution forecast. The Index of Dissimilarity (IOD) is widely used to measure allocation error (Duncan et al. 1961;Fonseca and Tayman 1989;Massey and Denton 1988). The IOD compares the percentage distribution of the forecast population by age group and the corresponding percentage distribution in the census. The IOD calculates the percentage that the forecast distribution would have to change to match the census distribution. The IOD ranges from 0 to 100%, with 0 indicating identical percentage distributions and 100 indicating a complete disparity between the forecast and census age distributions.

Data
Out of a universe of 3140 counties in the U.S, our data consist of 3106 counties or county equivalents whose geographic boundaries remained constant from 1990 to 2010. 2 These counties contain approximately 99% of the counties and the total population in the U.S. We collected 1990, 2000, and 2010 census data for 18 5-year age groups (0-4, 5-9…., 80-84, and 85 years and older) for males and females. Aside from boundary changes, implementing the H-P method is impacted by zero population counts. A CCR is undefined if the earlier census count (the denominator) is zero. At the county level of geography zero population counts are much less of a problem than for subcounty areas such as census tracts. We encountered only 68 1 The error distribution underlying the MAPE is often asymmetrical and right skewed, causing the MAPE to overstate the error represented by most of the observations. MAPE-R and the median absolute percent error (MEDAPE) are two measures of accuracy that have been used when the error distribution underlying the MAPE is highly asymmetrical (Baker et al. 2020;Swanson et al. 2011. Since forecast errors are generally stable across a variety of error measures (Rayer 2007), we forego using MEDAPE and MAPE-R in this study. 2 The 34 excluded areas are: 1) all 27 boroughs in Alaska; 2) Adams, Boulder, Broomfield, Jefferson, and Weld counties in Colorado; and 3) Alleghany and Halifax counties in Virginia. zero cells in the age and gender data for all three census points and assigned them a value of 1.

Total Population Forecast Error 3
For the analysis, we treat the UNCONST forecast as the baseline alternative that the other alternatives are compared to. This strategy provides a clearer focus on the efficacy of controlling and SYN relative to the most common application of the H-P method.
We begin the analysis by examining the forecast error for the total population. For UNCONST and UNSYN, their total population is a bottom-up number obtained by summing the age/gender forecasts. The controlled total population (CTRLPOP) is the independent population forecast for each county, which is the same for CTRLSYN and CTRLCONST. Table 1 shows UNCONST has less accuracy than UNSYN; the UNCONST MAPE (7.8%) is 1.0 percentage  Table 2 shows that UNCONST has fewer absolute percent errors for the total population under 10% (74.7%) compared to UNSYN (80.1%) and CTLPOP (83.9%). The opposite ranking of the alternatives occurs in all growth rate categories from 10% to 30+%. UNCONST has the most counties in these larger error categories, followed by UNSYN, and then CTRLPOP with the fewest counties.
We now examine the relative performance of the three alternatives in individual counties using a non-parametric approach that measures the number and percentage of counties where the absolute percent error (APE) in UNSYN is larger or smaller than the APE in UNCONST (See Table 3). UNSYN has a lower total population APE in 58.3% of the counties. Not only does UNSYN outperform UNCONST in more counties, but the difference in MAPEs where UNSYN outperforms is larger than the difference in MAPEs where UNCONST outperforms. In counties where UNSYN has a smaller APE than UNCONST, the CONST MAPE is 3.1 percentage points larger than the UNSYN MAPE. In counties where UNCONST has a smaller APE than UNSYN, the UNCONST MAPE is 2.3 percentage points smaller than the UNSYN MAPE.
To provide a geographic perspective of the total population forecast error, Appendix Tables 9 and 10 show the MAPE and MALPE for the counties in each state following the Table 1 format. In terms of accuracy (See Appendix Table 9), the UNCONST MAPE is larger than the UNSYN MAPE in 77% of the states (37), excluding the one state where the MAPEs are equal. CTRLPOP is the most accurate of any alternative across states. Compared to the CTRLPOP MAPE, the UNCONST MAPE is larger in 96% of the states (43), excluding the four states where the MAPEs are equal. The accuracy advantages of SYN and controlling over CONST are also evident by comparing the percent of states with "large" MAPEs that exceed 15% (10% of the states in UNCONST; 6% in UNSYN; and 2% in CTRLPOP). "Small" MAPEs (under 5%) occur in 22% of the states in UNCONST, 27% in UNSYN, and 39% in CTRLPOP.
Not only does UNSYN and CTRLPOP have greater accuracy than UNCONST for counties in more states, but the differences in MAPEs where UNSYN and CTRLPOP outperform are larger than the differences in MAPEs where UNCONST outperforms. The UNCONST MAPE is on average 1.5 percentage points higher in states with a lower UNSYN MAPE and − 0.2 of a percentage point lower in states with a higher UNSYN MAPE. The UNCONST MAPE is on average 2.9 percentage points higher in states with a lower CTRLPOP MAPE and − 0.6 of a percentage point lower in states with a higher CTRLPOP MAPE.
In terms of bias, UNCONST shows greater bias than UNSYN and CTRL-POP in more states, but there is greater variance in the differences in bias than in accuracy among the forecast alternatives (See Appendix Table 10). The UNCONST MALPE is larger than the UNSYN MALPE in 74% of the states. CTRLPOP is the least biased alternative across states. Compared to the CTRL-POP MALPE, the UNCONST MALPE is larger in 82% of the states. The bias advantages of SYN and controlling over CONST are also evident by comparing the percent of states with "large" MALPEs that exceed 6% (29% of the states in UNCONST; 8% in UNSYN; and 6% in CTRLPOP). "Small" MALPEs (under 2%) occur in 29% of the states in UNCONST, 59% in UNSYN, and 53% in CTRLPOP.
Not only do UNSYN and CTRPOP have less bias than UNCONST for counties in more states, but the differences in MALPEs where UNSYN and CTRL-POP outperform are larger than the differences in MALPEs where UNCONST outperforms. The UNCONST MALPE is on average 3.4 percentage points higher in states with a lower UNSYN MALPE and − 1.2 percentage points lower in states with a higher UNSYN MALPE. The UNCONST MALPE is on average 4.0 percentage points higher in states with a lower CTRLPOP MALPE and − 1.1 percentage points lower in states with a higher CTRLPOP MALPE.

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Using Synthetic Adjustments and Controlling to Improve County…

Forecast Error by Age Group
We constructed the forecasts using 17 5-year age groups and a terminal age group of 85 years and older, but evaluate forecast errors using a reduced set of seven groups. These groups cover the full age spectrum, adequately capture the age-specific performance of the alternatives, and make the analysis easier to follow. The seven age groups are younger than age 10 (hereafter < 10), 10-19, 20-34, 35-54, 55-64, 65-74, and 75 years and older (hereafter 75+). As a composite measure of the age group accuracy and bias, we also compute the average of the MAPEs and MALPEs across age groups.
For most age groups, UNCONST is less accurate than the other three forecast alternatives. (See Table 4). UNCONST has a higher MAPE than UNSYN in every age group and the age group average, except for ages 65-74 where the MAPEs are − 0.4 of a percentage point apart and ages under 10 where the MAPEs are equal. Similar patterns are seen in both controlled alternatives. UNCONST has a higher MAPE than CTRLCONST in every age group and the age group average, except for ages 65-74 where the MAPEs are − 0.4 of a percentage point apart and ages 75+ where the MAPEs are − 1.2 percentage points apart. The joint application of controlling and SYN (CTRLSYN) produces the most accurate forecasts by age group and the age group average. UNCONST has a higher MAPE than CTRLSYN in every age group and the age group average, except for ages 65-74 where the MAPEs are equal and ages 75+ where the MAPEs are − 0.6 of a percentage point apart. In general, the greatest benefit in terms of accuracy from SYN and controlling is seen in ages 10-54, which are typically most impacted by migration.
The patterns of forecast bias by age group are like those seen for forecast accuracy but the improvements in bias compared to UNCONST are somewhat larger. UNCONST has a higher MALPE than UNSYN in every age group and the age group average, except for ages 65-74 where the MAPEs are − 1.0 percentage point apart and ages under 10 where the MAPEs are − 1.7 percentage points apart. Similar patterns are seen in both controlled alternatives. UNCONST has a higher MALPE than CTRLCONST in every age group and the age group average, except for ages 65+ where the MALPEs are just over − 2.0 percentage points apart and ages < 10 where the MALPEs are − 1.2 percentage points apart. UNCONST has a higher MALPE than CTRLSYN in every age group and the age group average, except for ages 65-74 where the MALPEs are − 0.9 of a percentage point apart and ages < 10 where the MAPLEs are − 1.8 percentage points apart. The allocation error across age groups, as measured by the Index of Dissimilarity (IOD), is small and differs by only 0.1 of a percentage point between UNCONST and UNSYN. 4 Our last metric looks at the percentage of counties where the APE by age group and age group average is smaller for SYN compared to CONST for both the controlled and uncontrolled alternatives and the uncontrolled IOD. As Fig. 1 shows, the percentage is 50.0% or more in all but two comparisons (controlled ages 55-64 (48%) and uncontrolled ages 65-74 (42%) and it varies by age group. Excluding these two age groups, the percentage ranges from 51% for ages < 10 in the controlled alternative to 65% for ages 35-54 in the uncontrolled alternative. These percentages tend to be the highest for ages 10 to 54, the age group average, and the IOD and lowest in ages < 10 and 55-74.
To provide a geographic perspective of the forecast error for age groups, Appendix Table 11 shows the MAPE based on the average of the APEs across age groups for the counties in each state following Table 4 format. For most states, UNCONST has lower accuracy in its forecast by age than the other three alternatives. The UNCONST MAPE is larger than the UNSYN MAPE in 75% of the states, excluding the one state where the MAPEs are equal. The lower accuracy of UNCONST is more evident in the controlled alternatives. The UNCONST MAPE is larger than the CTRLCONST MAPE in 82% of the states and 86% of the states in CTRL-SYN. The accuracy disadvantage of UNCONST in forecasting age groups is also evident by comparing the percent of states with "large" MAPEs that exceed 15%. In UNCONST 11% percent of the states have large MAPEs compared to 6% for UNSYN and 4% for CTLRSYN. Only 2% of the states have large errors in CTRL-SYN. "Small" MAPEs (under 7.5%) occur in 33% of the states in UNCONST, the smallest percentage of any alternative. The percentage of "small" MAPEs ranges from 49% in CTRLCONST to 57% in CTRLSYN.
Not only does UNCONST have less accurate forecasts by age than the three other alternatives, but the differences in MAPEs where UNSYN, CTRLCONST,

Population Size
In this section, we analyze the error for the average of the APEs across the seven age groups and the IOD by population size and growth rate. Size represents the population in 2000 and the growth rate represents the percentage change from 1990 to 2000. This investigation examines the MAPE and IOD by size and growth rate categories for SYN and CONST under the uncontrolled and controlled alternatives, again using UNCONST as the basis for comparison. All four alternatives exhibit the well-known direct relationship between population size and forecast accuracy . As shown in Table 5, this direct relationship is not linear but tends to weaken or disappear once a certain size threshold is reached. UNCONST has lower accuracy, as measured by the MAPE, in most size categories compared to the other three alternatives. The main exception is for counties with 100,000-250,000 persons where the UNCONST MAPE is slightly lower at 5.6%; the MAPEs in this size category range from 5.7 to 5.9% in the other alternatives. The ability of SYN and controlling to improve accuracy is directly related to population size. While the percentage point differences between UNCONST and the other alternatives do not vary greatly by population size, the largest differences are seen in counties under 30,000 persons and range from 0.9 percentage points to 1.9 percentage points. For counties with 30,000 or more people the percent point differences, excluding signs, range from 0.0 percentage points to 0.7 percentage points. These results suggest that SYN and controlling are more useful at improving forecast accuracy for counties with less than 30,000 persons, which represent 59% of the counties in this study.
A stronger direct relationship is seen between population size and IOD. The ETA 2 falls into a narrow range of 0.215-0.235, which is higher than their counterparts (UNCONST and UNSYN) that range from 0.132 to 0.137. 5 However, there is very little difference in the IODs between UNCONST and UNSYN across size categories with percentage point differences, ignoring the sign, ranging from 0.0 to 0.2%.
Finally, we examine the percent of counties where the APE for the age group average and the uncontrolled IOD is smaller for SYN compared to CONST by population size category. As Table 6 shows, the percentages are 50% or more for every population size category, except for populations between 100,000 and 250,000 in Using Synthetic Adjustments and Controlling to Improve County… For the controlled APE and uncontrolled IOD, there is a weak relationship between population size and the percentage of counties where SYN outperforms CONST as shown in their small Tau-C values (0.030 controlled APE and − 0.043 uncontrolled IOD). 6 A stronger relationship (Tau-C = − 0.099) is seen in the uncontrolled APE alternative that shows SYN's outperformance decreases with population size. Also, controlling has an impact as seen by comparing the APE alternatives. The uncontrolled percentages are larger (better SYN performance) than the controlled percentages in counties with less than 30,000 persons, ranging between 3.5 percentage points and 13.5 percentage points. However, for counties with 50,000 or more people, the uncontrolled percentages are smaller, ranging from − 15.9 percentage points to − 12.3 percentage points.

Population Growth Rate
All four alternatives exhibit the well-known u-shaped relationship between population growth rate and forecast accuracy . As shown in Table 7, the fastest declining and growing areas tend to have higher MAPEs, ignoring the

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Using Synthetic Adjustments and Controlling to Improve County… sign, than areas with stable growth patterns. UNCONST has lower accuracy compared to the other three alternatives in most growth rate categories. The main exception is counties that experience a population loss. For the fastest declining counties (< − 5.0%), the UNCONST MAPE is slightly lower at 10.2%; the MAPEs in this growth rate category are 10.4 or 10.5% for the other alternatives. For the more slowly declining counties (− 5.0 to − 0.01%), the UNCONST MAPE (6.9%) is 0.1 of a percentage point lower than the MAPEs for CRTLCONST and CTRLSYN, but 0.1 of a percentage point higher than the MAPE for UNCTRLSYN. The ability of SYN and controlling to improve accuracy is directly related to the population growth rate. The numeric differences between UNCONST and the other alternatives widen consistently as the growth rate increases, with the largest differences seen when both controlling and SYN are employed. The difference between UNCONST and CTRLSYN is − 0.2 percentage points in the fastest declining counties and rises steadily reaching 4.8 percentage points in counties that grow by 30.0% or more. These results suggest that SYN and controlling will improve forecast accuracy the greatest in counties with increasing growth rates.
A weaker direct relationship is seen between growth rate and IOD. The ETA 2 falls into a narrow range of 0.087 and 0.089, which is lower than their counterparts (UNCONST and UNSYS) that range from 0.112 to 0.144. UNCONST has greater allocation error across age groups in all growth rate categories, except for the fastest declining counties where the IODs for UNCONST and UNSYN are 3.9% and 4.0%, respectively. However, there is very little difference in the IOD between UNCONST and UNSYN across growth rate categories with percentage point differences, ignoring the sign, ranging from 0.1 to 0.2%.
Finally, we examine the percent of counties where the APE for the age group average and IOD is smaller for SYN compared to CONST by population growth rate category. As Table 8 shows, the percentages are 50% or more for every population growth rate category, except for populations that declined more than − 5.0% in the uncontrolled IOD (48.7%). The other exceptions are for populations that declined less than − 5.0%, (uncontrolled APE (42.3%), controlled APE (48.6%), and uncontrolled IOD (45.8%)). Excluding these exceptions, the percentages range from 51.7% in the controlled APE alternative in counties with a growth rate under 5.0% to 72.3% in the uncontrolled APE alternative for counties that grew 30% or more. There is a direct relationship between the growth rate and the percentage of counties where SYN outperforms CONST. This relationship is stronger in both APE alternatives (Tau-C of 0.136 and 0.125, respectively for the uncontrolled and controlled alternatives) compared to 0.059 for the uncontrolled IOD. These findings further support that SYN will be more useful for improving forecast accuracy in counties with increasing growth rates. Controlling, however, has an adverse impact as seen by comparing the APE alternatives. The uncontrolled percentages are larger (better SYN performance) than the controlled percentages in all growth rate categories, except for counties that declined by less than − 5.0%, where the uncontrolled percentage was − 6.3 percentage points lower than the controlled percentage. For the other growth rate categories, the uncontrolled percentage exceeded the controlled percentage by between 1.0 percentage point (20.0 to − 29.9%) and 7.0 percentage points (0.0 to − 4.9%).

Summary
The objectives of this first-ever evaluation of the H-P method in counties nationwide were to more rigorously test the efficacy of using a synthetic method to adjust CCRs and CWRs (SYN) over the horizon rather than holding them constant (CONST), which is the usual approach when applying this method. We also examined the performance of SYN and CONST when the total population forecast was determined from the age-gender forecast (a bottom-up model) and when the age-gender forecasts were controlled to an independent county total population forecast. Out of a universe of 3,140 counties, we examined 3,106 counties that had constant boundaries between 1990 and 2010 and prepared a 10-year population forecast using a 2000 base year and a 2010 target year. We measured forecast accuracy, bias, and distributional error across age groups using the MAPE, MALPE, IOD, and the percentage of counties where the absolute percent forecast error (APE) for SYN was smaller than that for CONST.
Our main findings are: (1) SYN lowers forecast error compared to CONST whether the forecasts are controlled or not; (2) controlling also leads to the improvements in forecast error, often exceeding those seen in SYN; and (3) using both SYN and controlling together has the greatest effect in reducing forecast error. These findings remain after controlling for population size and growth rate, but the positive impacts on forecast error of SYN and controlling are most evident in counties with less than 30,000 population and that grow by 15% or more.
For the total population, UNSYN and CTRLPOP improve accuracy and reduce bias compared to UNCONST. The MAPEs for UNCONST, UNSYN, and CTLR-POP are 7.8%, 6.0%, and 6.0%, respectively. All three MAPEs show that the H-P method and the variants analyzed produce county-level total population forecasts that meet or exceed expected accuracy levels for 10-year county-level forecasts (Rayer et al. 2010;Smith et al. 2013, p. 365;Sprague 2013, p. 19). The MALPEs for UNCONST, UNSYN, and CTLRPOP are 4.3%, 2.5%, and 1.7%, respectively, which also indicate relatively low levels of bias in all alternatives.
In almost six out of ten counties, the UNSYN total population APE is smaller than the UNCONST APE and the accuracy gain in these counties (3.1 percentage points) is greater than the accuracy loss in the counties where the UNSYN APE is larger (− 2.3 percentage points). The advantages of UNSYN and CTRLPOP over UNCONST in reducing total population forecast errors are also widespread across the U.S. as demonstrated from the error patterns of counties within the 49 states analyzed in this paper.
The advantages of SYN over CONST in both the controlled and uncontrolled alternatives are also seen in the forecasts by age. The age-specific MAPEs for all four alternatives are in line with 10-year county forecasts produced using a modified Leslie Matrix model (Sprague 2013, p. 120) and are lower than 10-year forecasts produced for counties in Florida (Smith and Tayman 2003). The larger age-specific MAPEs in Florida's counties are likely a result of the high migration and faster growth rates relative to counties nationwide.
UNSYN has greater accuracy than UNCONST in all age groups, except for ages < 10 (where the MAPEs are equal) and in ages 65-74 where the UNSYN MAPE is 0.4 of a percentage point larger). In general, UNSYN and CTLRSYN (both controlling and SYN) increase accuracy the greatest in ages 10-54 where most migration usually occurs. Compared to CTRLSYN, UNCONST increases the MAPE by between 2.1 and 2.8 percentage points in this age range. This suggests that adjusting and controlling CCRs better reflects future migration processes rather than holding them constant. CTRLSYN is less effective in terms of accuracy for the two oldest age groups (65-74 and 75+). For ages 65-74, the CTRLSYN and UNCONST alternatives have the same MAPEs. For ages 75+, the UNCONST MAPE (6.5%) is − 0.6 of a percentage point lower than the CTRLSYN MAPE (7.1%).
The patterns of forecast bias by age group are like those seen for forecast accuracy but the improvements in bias compared to UNCONST are somewhat larger. Again, the greatest reduction in bias using SYN and controlling is in the peak migration ages. Compared to CTRLSYN, UNCONST increases the MALPE by between 3.0 and 6.0 percentage points in ages 10-54. CTRLSYN is less effective in terms of bias for ages < 10 and 65-74. For ages < 10 the UNCONST MALPE (0.6%) is − 1.8 percentage points lower than the CTRLSYN MALPE (2.4%), and for ages 65-74 the UNCONST MAPE (− 0.3%) is − 0.9 of a percentage point lower than the CTRL-SYN MAPE (− 1.2%).
The allocation error across age groups is low in both UNSYN and UNCONST (IODs under 3%), and UNCONST's IOD is only 0.1 percentage point less than UNSYN's average allocation across age groups The advantages of SYN and controlling over CONST in reducing population forecast error by age are also widespread across the U.S. as demonstrated from the MAPE of the average error across age groups of the counties within the 49 states analyzed in this paper.
We analyzed the average error across age groups by population size and growth rate. UNCONST has lower accuracy, as measured by the MAPE, in most size categories compared to the other three alternatives. While the percentage point differences between UNCONST and the other alternatives do not vary greatly by population size, SYN and controlling are more useful in improving forecast accuracy in counties with less than 30,000 persons.
In most growth rate categories, UNCONST has lower accuracy compared to the other three alternatives, as measured by the MAPE. The main exception is counties that experienced population losses, but the percentage point differences are small ranging between − 0.1% and − 0.3%. The standard H-P method applies a constant set of cohort growth rates to a beginning population, which can lead to a strong upward bias when applied to rapidly growing places. As such, it recommended that H-P forecasts of age and gender be controlled to an independent total population forecast (Baker et al. 2020;Smith et al. 2013, pp. 180-181;Swanson et al. 2010). Our analysis confirms the benefits of controlling as the numeric differences between UNCONST and the other alternatives widen consistently as the growth rate increases, indicating that SYN and controlling will improve forecast accuracy the greatest in counties with increasing growth rates.

Conclusion
The cohort-component method (CCM) is the most widely used approach for producing forecasts by age and sex and other demographic characteristics for counties and other higher-level geographies (Smith et al. 2013, p. 47). The CCM is very data intensive and requires at a minimum age-gender-specific death and migration rates and age-specific fertility rates. It also requires separate treatment of special populations such as the military, college students, and jails and prisons (Baker et al. 2017, p. 47). However, the CCM can track changes in the components of change over the forecast horizon and allow simulations showing the impacts on the future population of alternative assumptions for these components.
The H-P method and its alternatives studied here are low cost and substantially less data intensive than the CCM and produce forecasts of the total population and demographic characteristics with similar levels of forecast error compared to its more data-intensive cousin (Hauer 2019;Smith and Tayman 2003;Sprague 2013;Wilson 2016). We know of at least one state demographic center that formally did its county forecasts using the CCM and switched to the H-P method controlled to independent total population forecast using similar extrapolation methods shown in this paper (Rayer and Wang 2020). While there are purposes for which the H-P method will not be useful, it is a viable alternative for county forecasts when only information on a future population and its composition is needed and not information on the components of population change and their effects. The H-P method does not provide information on the components of population change because it uses CCRs that combine the effects of mortality and migration and CWRs that are very rough proxies to fertility rates.
While the standard H-P method (UNCONST) produces reasonable county-level forecasts of the total population and demographic characteristics, we have shown that H-P county-level forecasts are improved by adjusting the CCRs and CWRs using the synthetic method based on state trends over the horizon and by controlling the demographic characteristics to an independent total population in the county. Both modifications contribute to the decrease in forecast error compared to UNCONST that does not control to the total population of the county or modify the CCRs and CWRs. While H-P forecasts for counties can be improved by applying controlling and SYN separately, the best (lower error) forecasts occur when both controlling and SYN are applied together. We recommend applying this dual approach when using the H-P method to prepare county population forecasts.
Another advantage of controlling and SYN is that they are low-cost modifications to the H-P method and are relatively simple to apply, making them easily accessible to those preparing population forecasts for counties. While some might question the accuracy of county total population controls produced from extrapolation methods, the preponderance of evidence suggests that these methods can produce total population forecasts of comparable accuracy to those produced by more complication forecasting techniques (Green and Armstrong 2015;Hauer 2019;Rayer 2008;Smith et al. 2013, pp. 331-336). SYN requires state-level forecasts by age and gender and other characteristics if desired. Such forecasts are routinely prepared by most state population centers periodically after the latest census. Some agencies update their forecasts annually and others multiple times before the next census.
The H-P method has mainly been used for subcounty geographic areas where fertility, mortality, and migration are non-existent, unreliable, or difficult to obtain. A few studies have evaluated the impact of controlling and SYN at the census tract level. Baker et al. (2020) evaluated UNCONST and CTLRCONST and for census tracts nationwide, and Tayman and Swanson (2017) evaluated UNCONST and UNSYN for census tracts in the State of New Mexico. Given the synergies of controlling and SYN in reducing county forecast errors compared to UNCONST, it would be useful to study these synergies in broad-based samples of census tracts or even smaller geographic areas such as block groups and to investigate forecast horizons longer than 10 years.
Using Synthetic Adjustments and Controlling to Improve County…  Using Synthetic Adjustments and Controlling to Improve County…  Using Synthetic Adjustments and Controlling to Improve County…