Closing the loops

How do we strike the right balance among closed loops revealing all possible leverage points, exogenous drivers for fidelity and economy, and practical focus on feasible policies?

On the SDwisdom blog, Jack Homer wonders whether we should be a little less endogenous:

The party line is that a model’s boundary should be broad enough so that the system’s main observed behaviors—such as S-shaped growth, oscillation, or overshoot and decline—are fully explained by the model’s endogenous structure.  One should avoid the use of exogenous time series drivers, because they undermine the ability of the model to explain and to anticipate change.

I mostly agree with this view but want to offer a friendly amendment here.  In my experience with real-world clients, I have often encountered situations in which it makes sense to employ exogenous time series for the sake of completeness and realism.

My experience suggests that we should be less doctrinaire about the endogenous perspective and understand that “endogenous” is a relative thing.  No model can be all-encompassing and explain all observed behavior patterns.  That’s why we define a model relative to some subset of behaviors also known as the dynamic problem.  As long as the model adequately addresses the dynamic problem, it shouldn’t really matter if the model has some exogenous time series included to improve the model’s realism.

The counterpoint to this might be George Richardson’s excellent article on the value of an endogenous perspective:

But the foundation of systems thinking and system dynamics lies deeper than these and is often implicit or even ignored: it is the “endogenous point of view.” The paper will begin with historical background, clarify the endogenous point of view, illustrate with examples, and argue that the endogenous point of view is the sine%qua%non of systems approaches.

The dead buffalo model on the left in Richardson’s figure is perhaps too extreme. I think what Homer is really arguing for is a middle road that might look like this:

In other words, it’s a hybrid model, with an endogenous core, and a few exogenous drivers from beyond the model boundary. For a firm, this might mean that we model the interactions of marketing, production, human resources, etc. endogenously. But we leave the world crude price and GDP of France exogenous, because we have no plausible means to influence them.

Richardson points out, in the context of Urban Dynamics:

How might thinking exogenously have affected conclusions emerging from these urban models? Consider just one: suppose, as a number of critics of various system dynamics studies have suggested, we chose to use time series data for urban population projections. Suppose the data-based projections were very carefully developed by sophisticated statistical tools and econometric methods, and suppose those projections were fed into URBAN1 in place of the endogenous stock of population. To make the implications easiest to see, let’s suppose the base run of the model looks just as it did in Figure 5b. What would Figure 5c look like? Sadly, population would not show a bump as it did in 5c because the sophisticated exogenous time series data would not be influenced in the slightest by the changing conditions in the model. We would not see the compensating urban migration effects we see in Figure 5c, and we would miss the crucial conclusion that population and business construction dynamics would naturally compensate for the jobs program. We’d probably think it was a long term policy success, and we would be dramatically wrong.

Similarly, a key part of my dissertation critique of the DICE model was the idea that its exogenous representation of technology excluded a set competitive dynamics between fossil fuels and renewables that are crucial to understanding climate policy. In both cases, leaving even a few loops exogenous might dramatically alter policy recommendations.

In another article, Homer rejoins:

In 1977, my teacher, SD pioneer Ed Roberts, wrote an important paper called “Strategies for Effective Implementation of Complex Corporate Models”, based on his years of consulting experience.  When it comes to policy recommendations, he said, “the organizations’ ability to absorb the associated change must be assessed…The model-builder and the organization both profit from implementation of moderate change proposals leading to some successful results; both lose from grandiose plans which fail to be moved ahead.”

Without too much extra effort beyond what we already do in modeling, we could assess policy feasibility by studying the political situation and its dynamics, looking at trends in public opinion surveys and other indicator data, and talking to experts. We could then calculate the “feasibility-adjusted value” of a policy by multiplying its potential impact (as determined by our SD model) by its likelihood of enactment.

In other words, one might ask, what good is it to know the nature of the best endogenous policy if the key leverage points are beyond your control? Or, as economists put it, what good are first-best policies in a second-best world?

I think Richardson might respond along the lines of Pascal’s wager:

To the extent that we accept some exogeneity in our models, we accept our fate in the lower left box. That choice presumes that our best strategy is to do a good job of predicting and preparing – making the best of a bad situation, or living to fight another day. Is that really a good idea – can we know, or at least make a good guess, whether the true state of affairs is exogenous or endogenous? Richardson argues that if we don’t know which box we’re in, it’s best to choose the endogenous approach:

If one were to recast Table 4 as a decision tree, the decision to take an endogenous point of view in all circumstances would have the highest net payoff, at least in terms of happy faces and the real feelings they represent. An endogenous point of view is potentially empowering, and that feels good to us.

If the right (endogenous) side of the diagram is better in some sense, we should ask a more important question: can we change the state of affairs, so that we can move from the left quadrants to the right? This is related to the idea that we might need to change paradigms to change the system.

This perspective is especially critical for problems like climate change, where the space of politically feasible policies currently does not include anything that actually solves the problem. Treating the infeasible loops as exogenous drivers won’t help. We need to ask, what loops that are now beyond our control can be activated in order to reach a more attractive solution?

Maybe there really is no good solution to climate and other Limits problems. Then perhaps it would be optimal in some sense to take catastrophe as exogenous and use models to plan a personal path through the eye of the storm. Personally, I’m not ready to accept that. My modeling objective remains to die with my boots on tackling the biggest problems.

See also:

Closing loops – practicalities

Why the World Loves Open Loop Models

What kind of model should I use?

Why the World Loves Open Loop Models

Propaganda, for one thing.

A while back I made a video about spreadsheets, that makes some points about open-loop models vs. real, closed-loop dynamic models:

The short version is that people tend to build this:

when reality works like this:

I think there are some understandable reasons to prefer the first, simpler view:

  • Just understanding the dynamics of accumulation (here, the vehicle stock) may be mind-blowing enough without adding feedback complexity.
  • It’s a start, and certainly better than no model or extrapolation!

Some of the reasons these models get built are a little less appetizing:

  • In the short run, some loops really aren’t closed (though the short run is rarely as short as you think, and myopia gets you in trouble).
  • They’re quicker and cheaper to build (if you don’t mind less insight).
  • Dynamic modeling skills, both for construction and consumption, are not very widespread.
  • Open-loop models are easier to calibrate (but a calibration neglecting accumulation and feedback is likely bogus and misleading).
  • Open-loop models are easier to manipulate to produce a desired outcome.

I think the last point is key. At Ventana, we’ve discussed – only partly in jest – creating a “propaganda mode” for Vensim and Ventity. This would automate the discovery of a parameterization of a model that both fits history and makes a preferred policy optimal.

Perhaps the ultimate example of this is the RMSM model. 20 years ago, this was the World Bank’s preferred tool for country modeling. When Gerald Barney and Weishuang Qu replicated the model in Vensim, they discovered that is was full of disconnected trees of causality. That would permit creation of a scenario in which GDP growth marched along merrily without any water, for example. Politically, this was actually a feature, not a bug, because some users simply didn’t want to know that their pet project would displace a lot of people or destroy resources.

I think the solution here is to equip people to ask the right questions that close loops. Once there’s an appetite for dynamic, operational thinking, we can supply good modelers to provide the tools.

Big Ideas About Systems

A slide at ISSS wonders what the big ideas about systems are:

*

Here’s my take:

  • Stocks & flows – a.k.a. states and rates, levels and rates, integration, accumulation, delays – understanding these bathtub dynamics is absolutely central.
  • Feedback -positive and negative feedback, leading to exponential growth and decay and other simple or complex behaviors.
  • Emergence – including the idea that structure determines behavior, the iceberg, and more generally that complex, counter-intuitive patterns can emerge from simple structures.
  • Relationships – ranging from simple connections, to networks, to John Muir’s insight, “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.”
  • Randomness, risk and uncertainty – this does a disservice by condensing a large domain in its own right into an aspect of systems, but it’s certainly critical for understanding the nature of evidence and decision making.
  • Self-reference, e.g., autopoiesis and second-order cybernetics.
  • Evolution – population selection and modification by recombination, mutation and imitation.**
  • Models – recognizing that mental models, diagrams, archetypes and stories can only get you so far – eventually you need simulation and other formal tools.
  • Paradigms – in the sense in which Dana Meadows meant, “The mindset or paradigm out of which the system — its goals, power structure, rules, its culture — arises.”

If pressed for simplification, I’ll take stocks, flows and feedback. If you don’t have those, you don’t have much.

* h/t Angelika Schanda for posting the slide above in the SD Society Facebook group.

** Added following a suggestion by Gustavo Collantes on LinkedIn, which also mentioned learning. That’s an interesting case, because elements of learning are present in ordinary feedback loops, in evolution (imitation), and in self-reference (system redesign).

Why is national modeling hard?

If you’ve followed the work on the System Dynamics National Model, you know that it came to an end uncompleted. Yet, there is a vast amount of interesting structure in the model, and there have been many productive spinoffs from the work. How can this be?

I think there are several explanations. One is that the problem is intrinsically hard. Economies are big, and they operate at many scales. There are micro processes (firms investing in capacity and choosing technologies) but also evolutionary processes (firms getting it wrong die).

This means there’s no one to ask when you want to understand how things work. You can’t ask someone about their car fuel purchase habits and aggregate up to national energy intensity, because their understanding encompasses Ford vs. Chevy, not all the untried and future contingencies in the economic network beyond their limited sphere of influence.

You can’t ask the data. Data must always be interpreted through the lens of a model, and model structure is what we lack. If we had a lot more data, we might be able to infer more about the constraints on plausible structures, but economic data is pretty sparse compared to the number of constructs we need to understand.

In spite of this, dynamic general equilibrium models have managed to model whole economies anyway. Why have they succeeded? I think there are two answers. First, they cheat. They reduce all behavior to an optimization algorithm. That’s guaranteed to yield an answer, but whether that answer has any relevance to the real world is debatable. Second, they give answers that people who fund economic models like: the world is just fine as it is, externalities don’t exist, and all policy interventions are costly.

All this is not to say that we’ll never have useful national models; indeed we already have many models (including the DGEs) that are useful for some purposes. But we still have a long way to go before we have solid macrobehavior from microfoundations to inform policy broadly.

 

Challenges Sourcing Parameters for Dynamic Models

A colleague recently pointed me to this survey:

Estimating the price elasticity of fuel demand with stated preferences derived from a situational approach

It starts with a review of a variety of studies:

Table 1. Price elasticities of fuel demand reported in the literature, by average year of observation.

This is similar to other meta-analyses and surveys I’ve seen in the past. That means using it directly is potentially problematic. In a model, you’d typically plug the elasticity into something like the following:

Indicated fuel demand 
   = reference fuel demand * (price/reference price) ^ elasticity

You’d probably have the expression above embedded in a larger structure, with energy requirements embodied in the capital stock, and various market-clearing feedback loops (as below). The problem is that plugging the elasticities from the literature into a dynamic model involves several treacherous leaps.

First, do the parameter values even make sense? Notice in the results above that 33% of the long term estimates have magnitude < .3, overlapping the top 25% of the short term estimates. That’s a big red flag. Do they have conflicting definitions of “short” and “long”? Are there systematic methods problems?

Second, are they robust as you plan to use them? Many of the short term estimates have magnitude <<.1, meaning that a modest supply shock would cause fuel expenditures to exceed GDP. This is primarily problem with the equation above (but that’s likely similar to what was estimated). A better formulation would consider non-constant elasticity, but most likely the data is not informative about the extremes. One of the long term estimates is even positive – I’d be interested to see the rationale for that. Perhaps fuel is a luxury good?

Third, are the parameters any good? My guess is that some of these estimates are simply violating good practice for estimating dynamic systems. The real long term response involves a lot of lags on varying time scales, from annual (perceptions of prices and behavior change) to decadal (fleet turnover, moving, mode-switching) to longer (infrastructure and urban development). Almost certainly some of this is ignored in the estimate, meaning that the true magnitude of the long term response is understated.

Stated preference estimates avoid some problems, but create others. In the short run, people have a good sense of their options and responses. But in the long term, likely not: you’re essentially asking them to mentally simulate a complex system, evaluating options that may not even exist at present. Expert judgments are subject to some of the same limitations.

I think this explains why it’s possible to build a model that’s backed up with a lot of expertise and literature at every equation, that fails to reproduce the aggregate behavior of the system. Until you’ve spend time integrating components, reconciling conflicting definitions across domains, and revisiting open-loop estimates in a closed-loop context, you don’t have an internally consistent story. Getting to that is a big part of the value of dynamic modeling.

Towards Principles for Subscripting in Models

For many aspects of models, we have well-accepted rules that define good practice. All physical stocks must have first-order negative feedback on the outflow. Normalize your lookup tables. Thou shalt balance units.

In some areas, the rules haven’t been written down. Subscripts (arrays) are the poor stepchild of dynamic models. They simply didn’t exist when simulation languages emerged, and no one really thinks about them much. They’re treated as a utility, like memory allocation in C, rather than as a key part of the model architecture. I think that needs to change, so this post is attempt to write down some guidance. Consider it a work in progress; I’d be interested in your thoughts.

What’s the Question?

There are really two kinds of questions:

  • How much detail do you want in your model? This is just the age-old problem of aggregation, which I won’t rehash in this post.
  • How do the subscripts you’re using contribute to a transparent, operational description of the system?

It’s the latter I’m concerned with. In essence: how do you implement a given level of detail so that the array structure makes sense? Continue reading “Towards Principles for Subscripting in Models”

Who cares if your model is rubbish?

All models are wrong, so does it matter if your model is more wrong than necessary?

If you’re thinking “no,” you’re not alone (but you won’t like this blog). Some models are used successfully for pure propaganda. Like the Matrix, they can be convincing for those who don’t get too curious about what lies beneath. However, if that’s all we learn to do with models, we’re doomed. The real potential of models is for improving control, by making good contingent predictions of “what might happen if we do X.”

Doing the best possible job of that involves tradeoffs among scope, depth and quality, between formal and informal methods, and between time spent modeling and time spent on everything else – data collection, group process, and execution. It’s difficult to identify the best choices for a given messy situation.

You can hide ugly aspects of a model by embedding it in a fancy interface, or by showing confidence bounds on simulations without examining individual trajectories. But if you take the low (quality) road, you’re cheating yourself, your clients and the world out of a lot of good insight.

Specifically:

  • You’ll make bad decisions.
  • You won’t learn much, or at least you won’t learn much that’s right.
  • You’ll get into trouble when you attempt to reuse or extend the model later.
  • People will find out. Maybe. (Sadly, if the situation is complex enough, they won’t.) Eventually, this may affect your credibility.
  • You will get less recognition for your work. (Models that are too large, and insufficiently robust, are the primary failure mode in the papers I review.)
  • The process will destroy your soul, or at least your brain.

That last point is the dealbreaker for me. I’m into modeling for the occasional glimpses of truth and beauty. Without that, it’s no fun. Continue reading “Who cares if your model is rubbish?”

On Compounding

This is a brief techy note on compounding in models, prompted by some recent work on financial functions, i.e. compound interest. It’s something you probably know, but don’t think about much. That’s because it’s irrelevant most of the time, except once in a while when it decides to bite you.

Suppose you’re translating someone’s discrete time model, and you decide to translate it to continuous time, because Discrete Time Stinks. The original has:

bank balance(t+1) = bank balance(t) * (1+0.1)

So you translate as: Continue reading “On Compounding”

Confronting the dreaded blank canvas

Donella Meadows walks a class through the model conceptualization process in Creating Models from Scratch:

One interesting thing here is that she starts with a causal loop diagram. This is a case where there are some clear physical quantities of interest (people and mosquitoes), so I would probably have started with stocks and flows. (Or, I like to think I would.) But you never know how things are going to go – I can think of other situations where CLDs worked out better. The key is to stay flexible and switch methods as needed, and as the audience requires (see around 30:00 for the stock-flow implementation).

An application of model critique

This long post walks through a real-world critique of an interesting model – this year’s Dana Meadows Award winner.

Testing is the key to making a good model even better.

Model Quality: the High Road

Therefore, in the interest of continuous improvement, I’ll take a hard look at a very interesting model. To get in the spirit, you might want to take a look at How to Critique a Model and my video critique of World Dynamics. I’m taking this model apart not because it’s bad, but because it’s interesting and worth investigating. (Taking apart bad models is sometimes fun too, but the supply of them is overwhelming.)

Before digging in, let me point out that I have no particular expertise in this area, so my critique is purely technical.

Inspection

The model (originally in STELLA, translated to Vensim here) passed my initial sniff test – no strange formulations, ugly spaghetti, cryptic variable names, or unit errors in the original.

However, it turns out that STELLA’s unit checking is not very strict. For example, it permits:

LOG10(Free Cortisol)/LOG10(Ref Free Cortisol)

with cortisol in units of nmol. This is a conceptual error –  logarithms are fundamentally dimensionless. Fortunately, it’s without consequence for model behavior – it just scales the input to a lookup.

Here, a better normalization would be:

LOG10(Free Cortisol/Ref Free Cortisol)

In my translation, I didn’t fix these issues; I suppressed them with a “DMNL LOG10” macro that hides the warning.

STELLA also permits unnormalized lookups without issuing a warning (maybe this is a buried preference somewhere). This is not necessarily an error, but it’s not best practice. It may conceal errors, and makes analysis difficult (more on this below).

One reviewer pointed out that a number of physical processes in the model are represented by goal seeking structures – essentially SMOOTHs. Here, the number of glucocorticoid receptors adjusts toward a level indicated by cortisol levels:


This is basically a shorthand for a real physical process that must involve inflows and outflows, something like this:

The physical representation is potentially better because it’s more operational. It invites more thinking along the lines of “where do these receptors come from?” It exposes one important possibility: asymmetry. The process that increases GR numbers might have a different time constant from the process that decreases GR numbers. However, absent detailed information about GR regulation, I have no idea how to implement such a thing. Maybe no one does: my experience with biological models is that there are always many layers of complexity surrounding the system of interest, and the literature often just scratches the surface.

TIME STEP & PULSE

The first thing I test in most models is to vary TIME STEP to check stability. The usual trick is to halve TIME STEP and see if you get the same answer, but this model already runs for 92,160 time steps (128 steps per hour for 720 hours). I don’t see any delays that are small enough to require that, but you can’t always see implicit time constants in a model. Still, I wonder, could you get away with a coarser step?

If you double the TIME STEP, you immediately see differences:

This suggests that TIME STEP needs to be small (perhaps even smaller than its already-tiny value). But where does this come from? It turns out to be due to the test input. In the original, the external stress consists of a series of IF THEN ELSE statements, like:

IF((TIME>1) AND (TIME<1+Stress_Stimulus_Duration)) THEN (1) ELSE(0) + ...

I implemented this in the Vensim version via the PULSE TRAIN function. But there’s a small problem here: if the stress stimulus duration is not a power of 2, the effective width of the pulse will vary a little bit as you change the TIME STEP (assuming that it is a power of 2, as is usual to minimize roundoff error). That in turn means that the area under the curve of the stress perception inflow to the model varies slightly with the duration of the pulse.

Often, that won’t matter, but because this model has a numerically sensitive threshold, it matters a lot.

It turns out that if you renormalize the external stress input to deliver constant area under the curve (see the updated model for details), you can get away with a TIME STEP of .03125 – 4x bigger. I think one might carry this idea even further, and switch to RK4 Auto integration and make the test input smooth, but I haven’t tried that. Fortunately, all of this concerns the test input to the model alone; the dynamics are nearly unaffected.

Lookup Bounds

Next, I check runtime warnings. This model generates quite a few, all concerning lookups that are out of bounds, like:

WARNING: Lookup out of bounds at 24.125 In -#ProCyt Eff on TRP#- computing -ProCyt Eff on TRP-.

It might be OK to run off the ends of a lookup table, if the slope at the endpoints is zero. But I prefer to suppress these warnings by adding points to the ends of the lookup so that the needed domain is explicitly covered. Most of these turn out to be OK, but I modified them anyway to suppress the warnings:

A few cases are hard to reconcile without more knowledge than I have. For example:

Above, the GR function effect has a small discontinuity. Its input (GR function) seems to be bounded at one, but adding (1,1) to the lookup would cause a break in the slope. I prefer to leave such warnings in place for later review.

Extreme Conditions

My next probe of a model is generally random Synthesim overrides of key stocks and flows, to see whether the model is robust to extreme disturbances. Generally, I’d say that this model is unusually robust, in that it’s hard to get stocks to go negative or produce other undesirable behavior. That’s good. Part of the reason for this may be that many of the relationships in the model are sigmoids, and therefore bounded above and below.

Here’s one example of a test: I replicate the “probable depression” scenario from the paper, and increase the size of the one-time external stress to 100 (2x). Then I look at every stock in the model to see what happens (the stocks are the state of the system, so if you look at those, you know everything). This is easy in Vensim if you create an instance of the Strip Graph that shows all levels:

One thing jumps out at me: the initial response of serotonin is opposite the long term response:

This is not unusual, and it’s mentioned in the paper. However, I got curious about its origins, so I started causal tracing to identify the source of the behavior. The answer is … it’s complicated. But along the way, I do notice some fairly extreme nonlinear behaviors, like this:

Is this realistic, or is it a consequence of lookup table clipping and log(x)/log(y) normalizations? I can’t say for sure, but this tests the limits of what I perceive as reasonable behavior. But then, if systems don’t occasionally surprise you with weird (but real) behavior, you’re not paying attention. This is something I’d flag for further investigation.

Sensitivity

Ultimately, what we want out of this model is to identify interventions that can help people with immune/hormone/mood problems. The obvious way to get that advice out of the model is to do a lot of sensitivity analysis to test alternatives. Generically, we’re interested in two things:

  • Can you change the system state directly in some beneficial way, e.g. by administering a drug that supplies a hormone, or lowering stress?
  • Can you restructure the system, by changing parameters that govern the strength of feedback loops or adding/deleting links?

In a sense, these are all the same thing – a parameter is just a constant state that isn’t in the model (yet), and adding a link is like giving an implicit 0 parameter a nonzero value.

For the answers to make sense, you need a way to (a) influence each state, and (b) change the gain of each loop, preferably independently. In this model, that’s a bit tricky. Consider the effects of ProInflammatory Cytokines:

There are effects on stress, cortisol, and other things. Each has its own sigmoid-shaped lookup table transforming the input to the output. All the inputs are normalized to the same constant, Ref ProCyt. The normalization is good practice, but insufficient for testing purposes, because there’s no independent way to vary the gain on these loops by varying the shapes of the lookups. Yes, it’s possible to simply edit the curves, but that’s impractical for comprehensive, automated experimentation. Two approaches might be helpful:

1. Replace the lookups with parametric curves. Then the parameters can be varied to shift and stretch the functions. This is attractive because you get smooth behavior and a lot of flexibility. However, it’s a lot of work to implement. The functional forms may be arcane, and  you can’t easily visualize them until you run the model. Here are a few sigmoid options I’ve collected over the years:

:MACRO: SSHAPE3(x,slope,lowlim,uplim,x0)
SSHAPE3 = lowlim+(uplim-lowlim)*(1/(1+exp(-4*slope*xe))) ~ Dmnl ~ defaults: lowlim= 0; uplim = 1; slope = 1; x0=1 this gives a symmetric \ S-shape from lowlim to uplim through with 1 being the inflection point and \ derivative =  at this point = slope*(uplim-lowlim) |
xe = MAX(-ZIDZ(25,4*ABS(slope)),MIN(x-x0,ZIDZ(25,4*ABS(slope)))) ~ Dmnl ~ Clip to avoid floating point errors at extreme positive / negative values \ of x |
:END OF MACRO:

:MACRO: SSHAPE2(input)
SSHAPE2 = exp(MAX(-50,MIN(50,input)))/(1+exp(MAX(-50,MIN(50,input)))) ~ Dmnl ~ Exponential s-shaped curve; -infinity -> 0, 0 -> .5, infinity -> 1 |
:END OF MACRO:

:MACRO: SSHAPE(xin,profile)
SSHAPE = IF THEN ELSE( input>0.5, 1-(1-input)^profile*0.5/0.5^profile, input^profile*\ 0.5/0.5^profile) ~ Dmnl ~ S-shaped response, from 0-1 for input from 0-1. Profile should normally be >=1 \ (1=linear; 2=quadratic) Always passes through (0.5, 0.5) |
input = MIN(1,MAX(0,xin)) ~ xin ~ |
:END OF MACRO:

2. Apply scaling parameters around the lookups. For example, if you’re starting with:

output = lookup( input/reference input )

you can add:

output = reference output*lookup( input/reference input )^scale

or

output = reference output*( 1-scale + scale*lookup( input/reference input ))

These don’t give you full control over the upper and lower bounds, slopes and asymptotes of the table, and they might not work when the lookup doesn’t pass through some obvious point like (1,1) or (0,0). So, in some cases you may need to be cleverer, or to choose approach #1 instead.

STELLA lookups, and Vensim’s WITH LOOKUP function, don’t really lend themselves to this treatment – you have to add an additional variable to transform the output downstream of the lookup. That’s why I tend to prefer the original Vensim lookup syntax. However it’s implemented, I think some level of parametric control over lookup usage is essential.

After some noodling, I settled on the following policy:

  • For dimensionless parameters with (apparent) 0-1 bounds, or centered around 1, apply a scaling exponent, so y = y0*lookup(x/x0)^s
  • For parameters bounded below at 0, apply a scaling multiplier, so y = s*lookup(x/x0)
  • For parameters with log inputs, apply a shift of the input, so y = lookup(x/x0+s)
  • Where I couldn’t figure out what do do, or a loop already contains other independent scaling parameters, skip the item

With scaling parameters in place, I ran an all-constants sensitivity analysis on the model, testing the effect of 10% variations in each parameter against the integrated serotonin level over the simulation. I started from 2 cases: the “daily stress” scenario (repeated small events), and the “probable depression” scenario (one large stress event). I then sorted the results by rank of influence on serotonin:

These are interesting in several ways:

  • The model is only moderately sloppy – many parameters have a strong effect, especially in the Daily Stress scenario.
  • There are big differences in sensitivity between the two scenarios, even though they differ only in the test input. This suggests that policies might have to be tailored to the stressor, among other things.
  • Some of the scale parameters on lookups are near the top of the list, confirming that testing lookup tables matters.

From a policy standpoint, you have to know a little more to make sense of these. What matters is not so much the response to a 10% change, but the response to an X% change, where X is the amount you could plausibly move a parameter. For example, preventing degradation of glucocorticoid receptors is clearly important (Ref GR Deg Fraction, top of list). However, the corresponding Permanent Degeneration Time is at the bottom of the list, presumably because a 10% change from 10 hours has only a tiny effect on the time horizon of the simulation. One would have to be more ambitious than that, but it might still be important.

Bottom Line

While there are a few features that could be reexamined, this model stands up to hard use well. It would also have to pass the face validity test with people who actually know something about the system, but given the paper’s citation list, I would anticipate some success on that front.

I think there might be a lot of interesting policy implications lurking in this model, waiting for an intrepid explorer with more subject matter expertise than I have. I think the crucial point here is that the structure identifies a mechanism by which patient outcomes can be strongly path dependent, where positive feedback preserves a bad state long after harmful stimuli are removed. Among other things, this might explain why it’s so hard to treat such patients. That in turn could be a basis for something I’ve observed in the health system – that a lot of doctors find autoimmune diseases mysterious and frustrating, and respond with a variation on the fundamental attribution error – attributing bad outcomes to patient motivation when delayed, nonlinear feedback is responsible.