Published Papers by Jack B Homer
System Dynamics Review, 2019
John Sterman (2018) has done a masterful job of explaining where system dynamics stands after 60 ... more John Sterman (2018) has done a masterful job of explaining where system dynamics stands after 60 years and identifying opportunities for improved practice in the areas of data collection, parameter estimation, and model analysis. He has argued eloquently that carefully following the scientific method and using the best current tools is essential, because "it is the best way to keep from fooling ourselves". He closes with a challenge: "The need is great. The path may be difficult. Will we take it?" I applaud this call to rigor, which has been a long-time concern of mine,
Systems, 2022
Many believe good government to be essential for a nation’s progress, but, in fact, governance is... more Many believe good government to be essential for a nation’s progress, but, in fact, governance is a multidimensional concept with uncertain implications for economic development and global sustainability. The World Bank has tracked six country-level Worldwide Governance Indicators since 1996. Statistical regression analysis across 150 countries identified two of these indicators, Government Effectiveness and Regulatory Quality, that consistently help to explain changes in economic growth and CO2 emissions. The regression results provided the evidence needed to incorporate the effects of governance in an existing climate-population simulation model. Policy testing of the revised model led to findings about what improved governance can and cannot do. The testing suggested that the best combination of such improvements could boost progress on emissions reduction without hindering economic development—but not enough to strongly mitigate climate change. Achieving the double goal of econo...
Systems, 2022
We present a practical guide and step-by-step flowchart for establishing uncertainty in-tervals f... more We present a practical guide and step-by-step flowchart for establishing uncertainty in-tervals for key model outcomes in a simulation model in the face of uncertain parameters. The process start with Powell optimization to find a set of uncertain parameters (the optimum param-eter set or OPS) that minimize the model fitness error relative to historical data. Optimization also help in refinement of parameter uncertainty ranges. Next, traditional Monte Carlo (TMC) ran-domization or Markov Chain Monte Carlo (MCMC) is used to create a sample of parameter sets that fit the reference behavior data nearly as well as the OPS. Under the TMC method, the entire pa-rameter space is explored broadly with a large number of runs, and the results are sorted for se-lection of qualifying parameter sets (QPS) to ensure good fit and parameter distributions that are centrally located within the uncertainty ranges. In addition, the QPS outputs are graphed as sen-sitivity graphs or box-and-whisker plots ...
Systems, 2025
Multiple studies, starting with Preston's work in 1975, have suggested that gross domestic produc... more Multiple studies, starting with Preston's work in 1975, have suggested that gross domestic product per capita (GDPPC) is an important explanatory factor for understanding differentials in life expectancy at birth (LEB) in countries around the world. This proposition was tested in the present study using twoperiod cross-sectional regression across a large number of both advanced and developing countries and 16 socioeconomic factors including GDPPC. The best-performing regression equations in the periods around 2000 and 2015 included four to six of these factors (government effectiveness, safe sanitation, poverty, and contraception, plus, in the circa-2000 period, Gini index and CO2 emissions); perhaps surprisingly, these equations did not include GDPPC. The results were examined in greater detail for the world's 15 most populous countries, helping to identify key drivers of LEB growth for each of these countries from circa 2000 to 2015. The fact that GDPPC drops out of the best equations calls into question the view that economic growth is the correct primary target for nations seeking to increase their average life expectancy.
PLOS One, 2025
In America and around the world, one’s chances for well-being depend on systems that are not yet ... more In America and around the world, one’s chances for well-being depend on systems that are not yet built for everyone to thrive together. Knowing that life expectancy and life evaluation are far below their full potential, with stark injustices by race/ethnicity, we ask: how can the United States make a great stride toward multiracial well-being?. This study explores potential impacts of a federal plan for thriving people and places. We estimate the likely effects of 68 recommendations using ReThink Health’s Thriving Together Model (TTM), revised with new data and new features including a multisolving ratio that accounts for greater cost-effectiveness when a proposed action advances multiple goals at once. The TTM is a previously published system dynamics model that simulates changes over time when community assets (both funding and in-kind resources) are invested in four drivers of population well-being (i.e., Vital Conditions, Belonging and Civic Muscle, Fairness in System Design, and Urgent Services Capacity). All drivers work together through a dynamic structure that influences individual states of thriving, suffering, and life expectancy (overall and by race/ethnicity). The model specifies three reinforcing dynamics, including an “expanding the pie” loop that can increase available assets and improve all four well-being drivers over time. Results reveal a plausible scenario over 25 years in which thriving could rise 20 percentage points, suffering could drop 2.5 percentage points, and average life expectancy could grow by 2.6 years – all from equitable progress across racial/ethnic groups. Every subgroup improves, but the greatest gains would likely be among Black and Hispanic Americans. Sensitivity tests confirm that the model’s conclusions are robust across identified uncertainty ranges. The federal plan points the way toward a just transition for multiracial well-being. It does not require new appropriations or authorities: only the will and wherewithal to bring these recommendations to life.
Systems, Nov 21, 2024
US life expectancy now lags significantly behind the majority of high-income countries, having gr... more US life expectancy now lags significantly behind the majority of high-income countries, having grown more slowly since 1980 for reasons that are not evident and have been debated. An exploratory system dynamics model is presented that reproduces the full pattern of US life expectancy from 1960 to the present. Multiple socioeconomic and behavioral factors help to explain the historical pattern, two of them apparently most responsible for the stagnation since 1980: the growth of obesity and the leveling off of growth in social spending. Some of the factors in the model are traced back to earlier causes, and obesity’s growth in particular is traced back to excess growth in private health care spending and its adverse effect on workers’ wages. The model’s base run does a good job of reproducing a variety of historical time series data going back to the 1960s, and counterfactual testing produces plausible results. The model may thus be considered a reasonable starting point for more conclusive future modeling of US life expectancy.
Obesity Reviews, 2022
Adolescent overweight and obesity (AdOWOB) in Europe has proven to be a persistent and complex pr... more Adolescent overweight and obesity (AdOWOB) in Europe has proven to be a persistent and complex problem, and appropriate systems methods may help in evaluating potential policy options. This paper describes the development of a system dynamics model of AdOWOB as part of the EU-funded CO-CREATE project. The model was developed using literature and data from the Health Behavior in School-Aged Children (HBSC) study across 31 European countries. We identified 10 HBSC variables that were included as direct or indirect drivers of AdOWOB in the dynamic model, seven at the level of the individual, and three related to the social environment. The model was calibrated to 24 separate cases based on four gender and perceived wealth segments for each of the five CO-CREATE countries (The Netherlands, Norway, Poland, Portugal, and the UK) and for Europe overall. Out of 10 possible intervention points tested, exercise, fruit, life dissatisfaction, school pressure, and skipping breakfast were identified as the top five most influential ones across the 24 cases. These model-based priorities can be compared with the policy ideas suggested by the CO-CREATE adolescents.
System Dynamics Review, 2023
We recently published results from an SD model of adolescent overweight and obesity using data fr... more We recently published results from an SD model of adolescent overweight and obesity using data from 31 European countries that participate in the Health Behavior in School-Aged Children (HBSC) study. During model development, we sought to identify a feedback structure with high explanatory power that avoided speculative relationships. Expert reviewers generally agreed with our modeling decisions, but two decisions did raise questions: (1) excluding the influences of food environment and built environment, for which HBSC provided no data; and (2) including five causal links that were supported statistically but might be considered disputable. To address the reviewers' questions, we created four possible model structures and performed automated calibration followed by intervention testing and ranking. We then compared the goodness-of-fit and intervention results. We discuss implications for how to move forward with the model, including through additional data gathering.
Systems, 2022
We present a practical guide and step-by-step flowchart for establishing uncertainty intervals fo... more We present a practical guide and step-by-step flowchart for establishing uncertainty intervals for key model outcomes in a simulation model in the face of uncertain parameters. The process starts with Powell optimization to find a set of uncertain parameters (the optimum parameter set or OPS) that minimizes the model fitness error relative to historical data. Optimization also helps in refinement of parameter uncertainty ranges. Next, traditional Monte Carlo (TMC) randomization or Markov Chain Monte Carlo (MCMC) is used to create a sample of parameter sets that fit the reference behavior data nearly as well as the OPS. Under the TMC method, the entire parameter space is explored broadly with a large number of runs, and the results are sorted for selection of qualifying parameter sets (QPS) to ensure good fit and parameter distributions that are centrally located within the uncertainty ranges. In addition, the QPS outputs are graphed as sensitivity graphs or box-and-whisker plots for comparison with the historical data. Finally, alternative policies and scenarios are run against the OPS and all QPS, and uncertainty intervals are found for projected model outcomes. We illustrate the full parameter uncertainty approach with a (previously published) system dynamics model of the U.S. opioid epidemic, and demonstrate how it can enrich policy modeling results.
Systems, 2022
Local communities sometimes face severe shocks, such as the COVID-19 pandemic or economic recessi... more Local communities sometimes face severe shocks, such as the COVID-19 pandemic or economic recession, which inflict widespread harm, intensify injustice and test the ties that bind people together. A recent “Springboard” theory proposes a way to spring forward toward an equitable, thriving future by altering priorities among four structural drivers of population well-being: the extent of vital conditions, equity, urgent services capacity, and belonging and civic muscle. To explore the strategic implications of the Springboard theory, we developed the Thriving Together Model, a system dynamics simulation model that lets users play out alternative investment priorities and track changes over a decade as they try to maximize the number of people thriving and minimize suffering. The prototype model is exploratory, subject to further refinement and empirical support, but it has already sparked creative conversations among hundreds of changemakers who have interacted with it through an interactive theater. This paper presents the model’s structure, illustrative results, and tentative insights. The Thriving Together Model extends Ostrom’s Nobel Prize-winning work on shared stewardship by offering a general explanation about how stewards of a divided community can heal through a traumatic shock and spring forward toward a future with greater well-being and justice.
Systems, 2022
Many believe good government to be essential for a nation’s progress, but, in fact, governance is... more Many believe good government to be essential for a nation’s progress, but, in fact, governance is a multidimensional concept with uncertain implications for economic development and global sustainability. The World Bank has tracked six country-level Worldwide Governance Indicators since 1996. Statistical regression analysis across 150 countries identified two of these indicators, Government Effectiveness and Regulatory Quality, that consistently help to explain changes in economic
growth and CO2 emissions. The regression results provided the evidence needed to incorporate the effects of governance in an existing climate-population simulation model. Policy testing of the revised model led to findings about what improved governance can and cannot do. The testing suggested that the best combination of such improvements could boost progress on emissions reduction without hindering economic development—but not enough to strongly mitigate climate change. Achieving the double goal of economic development and strong climate change mitigation would thus require some kind of extra effort that does not fall under the usual definitions of good national governance.
Systems, 2022
The growing burden of chronic disease represents a complex challenge to public health. Innovative... more The growing burden of chronic disease represents a complex challenge to public health. Innovative approaches, such as system dynamics simulation modeling, can aid public health professionals in understanding such complex issues and identifying effective solutions. This paper describes a system dynamics model and its application in projecting the impacts of evidence-based inter-ventions on chronic disease for the state of Colorado. The development of the model was guided by data and input from subject matter expertise, peer-reviewed literature, and surveillance data. The model includes 28 intervention levers for chronic disease prevention, screening, and management. Interventions were simulated from 2020 to 2050 to project their impact on ten preventable causes of death. The simulations indicated the 6 most impactful interventions by 2050 to be adult smoking prevention, diabetes prevention, smoking cessation, blood pressure management, adult physical activity promotion, and colorectal cancer screening. Together, these 6 interventions could reduce preventable deaths by 7.1%, or 74% of the 9.6% reduction from all 28 interventions combined. This system dynamics model is a flexible tool that could be adapted or extended to include other populations or preventable chronic diseases. Prioritization and wide-scale implementation of the most impactful interventions could significantly reduce preventable deaths resulting from chronic disease.
System Dynamics Review, 2021
My purpose here is to explore the extent to which feedbacks from climate change are likely to aff... more My purpose here is to explore the extent to which feedbacks from climate change are likely to affect (either directly through fatalities or through adverse effects on the economy) such key population metrics as life expectancy and years of potential life lost. This is not meant to be a definitive analysis but is rather a first step that may help global modelers close the loop from climate change back to human welfare.
Chapter 13, Complex Systems and Population Health, Oxford U Press, 2020
The Rethink Health Dynamics Model represents the complex dynamics of a regional health system in ... more The Rethink Health Dynamics Model represents the complex dynamics of a regional health system in the United States and has been calibrated for more than 10 regions using nationwide and local data. When testing single interventions, the simulated improvement in system performance is often less than desired. By experimenting with combinations of interventions, we authors have identified recurring reasons for underperformance, or intervention pitfalls. Here we discuss four common pitfalls and possible ways to overcome each with additional intervention. The pitfalls include (a) trying to cut costs without changing payment incentives; (b) depleting available funds without securing sustainable financing; (c) trying to achieve greater equity through service delivery without building capacity to meet greater demand; and (d) missing the opportunity to achieve multiple goals simultaneously through the use of mutually supporting interventions. We illustrate each pitfall and proposed solution with causal feedback diagrams and simulation output graphs.
Milbank Quarterly, 2020
Decision-makers face a necessary but difficult task to prioritize interventions with the greatest... more Decision-makers face a necessary but difficult task to prioritize interventions with the greatest potential to produce equitable health and well-being. Using County Health Rankings data for a pre-defined peer group of 39 urban U.S. counties, we performed statistical regressions to identify 37 cross-impacts among 15 threats to health and well-being. Adding appropriate time delays, we developed a dynamic model of these cross-impacts and simulated each of the counties over 20 years to assess the likely impact of 12 potential interventions—individually and in a combined portfolio—for three outcomes: (1) years of potential life lost; (2) fraction of adults in fair-poor health; and (3) total spending on urgent services.
The combined portfolio yielded improvements by year 20 that are considerably greater than those at year 5, indicating that the time delays have a major effect. Despite wide variation in threat levels across counties, the list of top-ranked interventions is strikingly similar. Poverty reduction and social support came out as the most highly ranked interventions, even in the shorter term, for all outcomes in all counties. Interventions affecting smoking, addiction, routine care, health insurance, violent crime, and youth education were also important contributors to certain outcomes but not others.
Complex Systems and Population Health: A Primer (Oxford U Press), 2020
The US health system has repeatedly resisted attempts at improvement. The Rethink Health Dynamics... more The US health system has repeatedly resisted attempts at improvement. The Rethink Health Dynamics Model model focuses on health reform within any designated region of the US, and is a descendant of the CDC's "HealthBound" model of national health reform. We have previously described how the Rethink Health model can be used for policy analysis, learning, and helping local leaders in their collective endeavors. But we have not shown how its dynamic structure can be used to diagnose and suggest solutions to various kinds of policy resistance. In this article we go "under the hood" of the model to see how it can explain, in clear structural terms, why well-intended interventions sometimes fall short, as well as what more can be done to overcome those pitfalls and improve system performance.
System Dynamics Review, 2019
Our models should be judged based on their level of evidence for both detailed structure (how the... more Our models should be judged based on their level of evidence for both detailed structure (how the system works) and behavior (what has actually happened over time). Anyone regardless of technical background can make a greater effort to support their models with historical time series along multiple dimensions. Clients, audiences, and reviewers of submitted papers should expect at least this level of rigor from mature models; and models that lack it should be designated by conferences and journals as exploratory and having a lower level of evidence. I see no reason we cannot take this step along John’s path immediately.
System Dynamics Review, 2019
I describe here an approach to the building and testing of models which has worked well with clie... more I describe here an approach to the building and testing of models which has worked well with clients, audiences, and scholarly journals throughout my career as an SD modeler. This approach revolves around the careful gathering of evidence for both system structure and behavior. It may not be right for everyone, but I do think it has the advantage of being more streamlined and directed than traditional descriptions of the modeling process, while maintaining essential rigor. It may help people with basic SD training to develop scientifically stronger models, and may also make our approach more understandable for people inside and outside the SD field.
System Dynamics Review, 2015
System Dynamics Review, Jan 1, 2001
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Published Papers by Jack B Homer
growth and CO2 emissions. The regression results provided the evidence needed to incorporate the effects of governance in an existing climate-population simulation model. Policy testing of the revised model led to findings about what improved governance can and cannot do. The testing suggested that the best combination of such improvements could boost progress on emissions reduction without hindering economic development—but not enough to strongly mitigate climate change. Achieving the double goal of economic development and strong climate change mitigation would thus require some kind of extra effort that does not fall under the usual definitions of good national governance.
The combined portfolio yielded improvements by year 20 that are considerably greater than those at year 5, indicating that the time delays have a major effect. Despite wide variation in threat levels across counties, the list of top-ranked interventions is strikingly similar. Poverty reduction and social support came out as the most highly ranked interventions, even in the shorter term, for all outcomes in all counties. Interventions affecting smoking, addiction, routine care, health insurance, violent crime, and youth education were also important contributors to certain outcomes but not others.