Category: SystemDynamics

Quite a while ago, I wrote about modeling the STEM workforce:

An integrated model needs three things: what, how, and why. The “what” is the state of the system – stocks of students, workers, teachers, etc. in each part of the system. Typically this is readily available – Census, NSF and AAAS do a good job of curating such data. The “how” is the flows that change the state. There’s not as much data on this, but at least there’s good tracking of graduation rates in various fields, and the flows actually integrate to the stocks. Outside the educational system, it’s tough to understand the matrix of flows among fields and economic sectors, and surprisingly difficult even to get decent measurements of attrition from a single organization’s personnel records. The glaring omission is the “why” – the decision points that govern the aggregate flows. Why do kids drop out of science? What attracts engineers to government service, or the finance sector, or leads them to retire at a given age? I’m sure there are lots of researchers who know a lot about these questions in small spheres, but there’s almost nothing about the “why” questions that’s usable in an integrated model.

I think the current situation is a result of practicality rather than a fundamental philosophical preference for analysis over synthesis. It’s just easier to create, fund and execute standalone micro research than it is to build integrated models.

According to Jay Forrester, Gordon Brown said it much more succinctly:

The message is in the feedback, and the feedback is inherently
interdisciplinary.

In my recent Data & Uncertainty talk, one slide augmented Barry Richmond’s list of 7 critical modes of thinking:

The four new items are Statistical, Solution, Behavioral, and Complexity thinking. The focus on solutions and behavioral decision making has been around for a long time in SD (and BDM is really part of Barry’s Operational Thinking).

On the other hand, statistical and complexity elements are not particularly widespread in SD. Certainly elements of both have been around from the beginning, but others – like explicit treatment of measurement errors, process noise and Bayesian SD (statistical) and spatial, agent and network dynamics (complexity) are new. Both perhaps deserve some expansion into multiple concepts, but it would be neat to have a compact list of the most essential thinking modes across disciplines. What’s your list?

Following my big tent query, I was reexamining Axtell’s critique of SD aggregation and my response. My opinion hasn’t changed much: I still think Axtell’s critique of aggregation is very useful, albeit directed at a straw dog vision of SD that doesn’t exist, and that building bridges remains important.

As I was attempting to relocate the critique document, I ran across this nice article on Eight grand challenges in socio-environmental systems modeling.

Modeling is essential to characterize and explore complex societal and environmental issues in systematic and collaborative ways. Socio-environmental systems (SES) modeling integrates knowledge and perspectives into conceptual and computational tools that explicitly recognize how human decisions affect the environment. Depending on the modeling purpose, many SES modelers also realize that involvement of stakeholders and experts is fundamental to support social learning and decision-making processes for achieving improved environmental and social outcomes. The contribution of this paper lies in identifying and formulating grand challenges that need to be overcome to accelerate the development and adaptation of SES modeling. Eight challenges are delineated: bridging epistemologies across disciplines; multi-dimensional uncertainty assessment and management; scales and scaling issues; combining qualitative and quantitative methods and data; furthering the adoption and impacts of SES modeling on policy; capturing structural changes; representing human dimensions in SES; and leveraging new data types and sources. These challenges limit our ability to effectively use SES modeling to provide the knowledge and information essential for supporting decision making. Whereas some of these challenges are not unique to SES modeling and may be pervasive in other scientific fields, they still act as barriers as well as research opportunities for the SES modeling community. For each challenge, we outline basic steps that can be taken to surmount the underpinning barriers. Thus, the paper identifies priority research areas in SES modeling, chiefly related to progressing modeling products, processes and practices.

Elsawah et al., 2020

The findings are nicely summarized in Figure 1:

Click to Enlarge

Not surprisingly, item #1 is … building bridges. This is why I’m more of a “big tent” guy. Is systems thinking a subset of system dynamics, or is system dynamics a subset of systems thinking? I think the appropriate answer is, “who cares?” Such disciplinary fence-building is occasionally informative, but more often needlessly divisive and useless for solving real-world problems.

It’s interesting to contrast this with George Richardson’s list for SD:

The potential pitfalls of our current successes suggest the time is right to sketch a view of outstanding problems in the field of system dynamics, to focus the attention of people in the field on especially promising or especially problematic issues. …

Understanding model behavior
Accumulating wise practice
Advancing practice
Accumulating results
Making models accessible
Qualitative mapping and formal modeling
Widening the base
Confidence and validation

Problems for the Future of System Dynamics
George P. Richardson

The contrasts here are interesting. Elsewah et al. are more interested in multiscale phenomena, data, uncertainty and systemic change (#5, which I think means autopoeisis, not merely change over time). I think these are all important and perhaps underappreciated priorities for the future of SD as well. Richardson on the other hand is more interested in validation and understanding of models, making progress cumulative, and widening participation in several ways.

More importantly, I think there’s really a lot of overlap – in fact I don’t think either party would disagree with anything on the other’s list. In particular, both support mixed qualitative and computational methods and increasing the influence of models.

I think Forrester’s view on influence is illuminating:

One hears repeatedly the question of how we in system dynamics might reach “decision makers.” With respect to the important questions, there are no decision makers. Those at the top of a hierarchy only appear to have influence. They can act on small questions and small deviations from current practice, but they are subservient to the constituencies that support them. This is true in both government and in corporations. The big issues cannot be dealt with in the realm of small decisions. If you want to nudge a small change in government, you can apply systems thinking logic, or draw a few causal loop diagrams, or hire a lobbyist, or bribe the right people. However, solutions to the most important sources of social discontent require reversing cherished policies that are causing the trouble. There are no decision makers with the power and courage to reverse ingrained policies that would be directly contrary to public expectations. Before one can hope to influence government, one must build the public constituency to support policy reversals.

System Dynamics—the Next Fifty Years
Jay W. Forrester

This neatly explains Forrester’s emphasis on education as a prerequisite for change. Richardson may agree, because this is essentially “widening the base” and “making models accessible”. My first impression was that Elsawah et al. were taking more of a “modeling priesthood” view of things, but in the end they write:

New kinds of interactive interfaces are also needed to help stakeholders access models, be it to make sense of simulation results (e.g. through monetization of values or other forms of impact representation), to shape assumptions and inputs in model development and scenario building, and to actively negotiate around inevitable conflicts and tradeoffs. The role of stakeholders should be much more expansive than a passive from experts, and rather is a co-creator of models, knowledge and solutions.

Where I sit in post-covid America, with atavistic desires for simpler times that never existed looming large in politics, broadening the base for model participation seems more important than ever. It’s just a bit daunting to compare the long time constant on learning with the short fuse on some of the big problems we hope these grand challenges will solve.

In a breakout in the student colloquium at ISDC 2022, we discussed the difficulty of getting a paper accepted into the conference, where the content was substantially a discrete event or agent simulation. Readers may know that I’m not automatically a fan of discrete models. Discrete time stinks. However, I think “discreteness” itself is not the enemy – it’s just that the way people approach some discrete models is bad, and continuous is often a good way to start.

On the flip side, there are certainly cases in which it’s sensible to start with a more granular, detailed model. In fact there are cases in which nonlinearity makes correct aggregation impossible in principle. This may not require going all the way to a discrete, agent model, but I think there’s a compelling case for the existence of systems in which the only good model is not a classic continuous time, aggregate, continuous value model. In between, there are also cases in which it may be practical to aggregate, but you don’t know how to do it a priori. In such cases, it’s useful to compare aggregate models with underlying detailed models to see what the aggregation rules should be, and to know where they break down.

I guess this is a long way of saying that I favor a “big tent” interpretation of System Dynamics. We should be considering models broadly, with the goal of understanding complex systems irrespective of methodological limits. We should go where operational thinking takes us, even if it’s not continuous.

This doesn’t mean that everything is System Dynamics. I think there are lots of things that should generally be excluded. In particular, anything that lacks dynamics – at a minimum pure stock accumulation, but usually also feedback – doesn’t make the cut. While I think that good SD is almost always at the intersection of behavior and physics, we sometimes have nonbehavioral models at the conference, i.e. models that lack humans, and that’s OK because there are some interesting opportunities for cross-fertilization. But I would exclude models that address human phenomena, but with the kind of delusional behavioral model that you get when you assume perfect information, as in much of economics.

I think a more difficult question is, where should we draw the line between System Dynamics and model-free Systems Thinking? I think we do want some model-free work, because it’s the gateway drug, and often influential. But it’s also high risk, in the sense that it may involve drawing conclusions about behavior from complex maps, where we’ve known from the beginning that no one can intuitively solve a 10th order system. I think preserving the core of the SD genome, that conclusions should emerge from replicable, transparent, realistic simulations, is absolutely essential.

Related:

Discrete Time Stinks

Dynamics of the last Twinkie

Bernoulli and Poisson are in a bar …

Modeling Discrete & Stochastic Events in Vensim

IM PRETTY SURE THIS IS THE FURST EVAH SYSTEM DYNAMICZ SIMULASHUN MODEL WRITTEN IN LOLCODE.

HAI 1.2
    VISIBLE "HAI, JWF!"
    
    OBTW
     ==========================================================================
     SYSTEM DYNAMICZ INVENTORY MODEL IN LOLCODE
     TOM FIDDAMAN, METASD.COM, 2021
     INSPIRED BY THE CLASSIC BEER GAME
     AND MODEL 3.10 OF MICHAEL GOODMAN'S 
     'STUDY NOTES IN SYSTEM DYNAMICS'
     ==========================================================================
    TLDR
    
    BTW FUNKTION 4 INTEGRATIN STOCKZ WITH NET FLOW INOUT
    HOW IZ I INTEGRATIN YR STOCK AN YR INOUT AN YR TIMESTEP
        FOUND YR SUM OF STOCK AN PRODUKT OF INOUT AN TIMESTEP
    IF U SAY SO
    
    BTW FUNKTION 4 CHARACTER PLOTZ
    HOW IZ I PLOTTIN YR X AN YR SYMBOL
        I HAS A STRING ITZ ""
        I HAS A COUNT ITZ 0
        IM IN YR XLOOP
            BOTH SAEM COUNT AN BIGGR OF COUNT AN X, O RLY?
                YA RLY, GTFO
                NO WAI, STRING R SMOOSH " " STRING MKAY
            OIC
            COUNT R SUM OF COUNT AN 1
        IM OUTTA YR XLOOP
        VISIBLE SMOOSH STRING SYMBOL MKAY
    IF U SAY SO
    
    BTW INISHUL TIME - DEKLARE SUM VARIABLZ AND INIT STOCKZ

    I HAS A INV ITZ 0.0         BTW INVENTORY (WIDGETS)
    I HAS A MAKIN               BTW PRODUCTION RATE (WIDGETS/WEEK)
    I HAS A SELLIN              BTW SALES RATE (WIDGETS/WEEK)
    I HAS A TIME ITZ 0.0        BTW LOL I WISH (WEEK)
    I HAS A TIMESTEP ITZ 1.0    BTW SIMULATION TIME STEP (WEEK)
    I HAS A ZEND ITZ 50.0       BTW FINAL TIME OF THE SIM (WEEK)
    I HAS A TARGET ITZ 20.0     BTW DESIRED INVENTORY (WIDGETS)
    I HAS A ADJTIME ITZ 4.0     BTW INVENTORY ADJUSTMENT TIME (WEEK)
    I HAS A ORDERIN             BTW ORDER RATE (WIDGETS/WEEK)
    I HAS A INIORDERS ITZ 10.0  BTW INITIAL ORDER RATE (WIDGETS/WEEK)
    I HAS A STEPTIME ITZ 30.0   BTW TIME OF STEP IN ORDERS (WEEK)
    I HAS A STEPSIZE ITZ 5.0    BTW SIZE OF STEP IN ORDERS (WIDGETS/WEEK)
    I HAS A INVADJ              BTW INVENTORY ADJUSTMENT NEEDED (WIDGETS)
    I HAS A WIP ITZ 0.0         BTW WORK IN PROGRESS INVENTORY (WIDGETS)
    I HAS A SHIPPIN             BTW DELIVERIES FROM WIP (WIDGETS/WEEK)
    I HAS A PRODTIME ITZ 4.0    BTW TIME TO PRODUCE (WEEK)
    
    VISIBLE "SHOWIN RESULTZ FOR PRODUKSHUN"
    
    IM IN YR SIMLOOP        BTW MAIN SIMULASHUN LOOP
        
        BTW CALCULATE RATES AND AUXILIARIES
        
        BTW STEP IN CUSTOMER ORDERS
        BOTH SAEM TIME AN BIGGR OF TIME AN STEPTIME, O RLY?
            YA RLY, ORDERIN R SUM OF INIORDERS AN STEPSIZE
            NO WAI, ORDERIN R INIORDERS
        OIC
        
        SELLIN R SMALLR OF ORDERIN AN QUOSHUNT OF INV AN TIMESTEP
        INVADJ R DIFF OF TARGET AN INV
        MAKIN R SUM OF SELLIN AN QUOSHUNT OF INVADJ AN ADJTIME
        MAKIN R BIGGR OF MAKIN AN 0.0
        SHIPPIN R QUOSHUNT OF WIP AN PRODTIME
        
        BTW PLOT
        VISIBLE SMOOSH TIME " " MAKIN MKAY
        BTW PRODUKT WITH SCALE FACTOR FOR SIZING
        I IZ PLOTTIN YR PRODUKT OF MAKIN AN 4.0 AN YR "+" MKAY
                
        BTW INTEGRATE STOCKS
        
        TIME R I IZ INTEGRATIN YR TIME AN YR 1.0 AN YR TIMESTEP MKAY
        INV R I IZ INTEGRATIN YR INV AN YR DIFF OF SHIPPIN AN SELLIN AN YR TIMESTEP MKAY
        WIP R I IZ INTEGRATIN YR WIP AN YR DIFF OF MAKIN AN SHIPPIN AN YR TIMESTEP MKAY
        
        BTW CHECK STOPPING CONDISHUN
        BOTH SAEM TIME AN BIGGR OF TIME AN SUM OF ZEND AN TIMESTEP, O RLY?
            YA RLY, GTFO
        OIC
        
    IM OUTTA YR SIMLOOP
    
    
KTHXBYE

YOU CAN RUN IT IN THE TUTORIALSPOINT ONLINE INTERPRETER, OR GET JUSTIN MEZA’S DESKTOP LCI.

SD INVENTORY LOLCODE.TXT

$lci main.lo
HAI, JWF!
SHOWIN RESULTZ FOR PRODUKSHUN
0.00 5.00
                    +
1.00 5.00
                    +
2.00 5.93
                        +
3.00 6.64
                           +
4.00 7.34
                              +
5.00 8.00
                                 +
6.00 8.62
                                   +
7.00 9.22
                                     +
8.00 9.78
                                        +
9.00 10.31
                                          +
10.00 10.82
                                            +
11.00 11.30
                                              +
12.00 11.75
                                                +
13.00 12.18
                                                 +
14.00 12.46
                                                  +
15.00 12.30
                                                  +
16.00 12.03
                                                 +
17.00 11.68
                                               +
18.00 11.29
                                              +
19.00 10.89
                                            +
20.00 10.51
                                           +
21.00 10.17
                                         +
22.00 9.89
                                        +
23.00 9.66
                                       +
24.00 9.49
                                      +
25.00 9.39
                                      +
26.00 9.35
                                      +
27.00 9.35
                                      +
28.00 9.40
                                      +
29.00 9.47
                                      +
30.00 14.56
                                                           +
31.00 15.91
                                                                +
32.00 14.12
                                                         +
33.00 14.07
                                                         +
34.00 14.45
                                                          +
35.00 14.73
                                                           +
36.00 15.01
                                                             +
37.00 15.27
                                                              +
38.00 15.51
                                                               +
39.00 15.75
                                                                +
40.00 15.97

I THINK THIS SHOULD BE A PART OF EVERY SYSTEM THINKERZ LITTERBOX TOOLBOX.