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The evolution of empirical ABMs

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A talk at the workshop on "Agent-Based Models in Philosophy: Prospects and Limitations", Rurh University, Bochum, Germany

Abstract:
ABMs (like other kinds of model) can be used in a purely abstract way, as a kind of thought experiment - a way of thinking about some aspect of the world that is too complicated to hold in our mind (in all its detail). In this way it both informs and complements discursive thought. However there is another set of uses for ABMs - empirical uses - where the mapping between the model and sets of observation-derived data are crucial. For these uses, one has to (a) use the mapping to get from some data to the model (b) use the model for some inference and (c) use the mapping again back to data. This includes both predictive and explanatory uses of ABMs. These are easily distinguishable from abstact uses becuase there is a fixed and well-defined relationship between the model and the data, this is not flexible on a case by case basis. In these cases the reliability comes from the composite (a)-(b)-(c) mapping, so that simplifying step (b) can be counterproductive if that means weakening steps (a) and (c) because it is the strength of the overall chain that is important. Taking the use of models in quantum mechanics as an example, one can see that sometimes the evolution of the formal models driven by empirical adequacy can be more important than the attendent abstract models used to get a feel for what is happening. Although using ABM's for empirical purposes is more challenging than for purely abstract purposes, they are being increasingly used for empirical explanation rather than thought experiments, and there is no reason to suppose that robust empirical adequacy is unachievable.

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The evolution of empirical ABMs

  1. 1. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 1 The Possible Evolution of Empirical ABMs vs the danger of getting what you like Bruce Edmonds Centre for Policy Modelling Manchester Metropolitan University
  2. 2. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 2 Models as Tools • Models must be about something (or they are not a model) but there are different ways they can relate to whatever it is they are modelling, some of which are very indirect • Although we can often think of a model as a kind of picture – the correspondence idea of modelling – it is often more useful to think of them as a tool • Example: the gas law equations that predict temperature, pressure etc. of a gas • Example: Schelling’s model of segregation which is about an idea – a counterexample to an assumption – not directly about anything observed
  3. 3. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 3 Models as Specialised Tools • “Model” is a very generic category, covering many different kinds of construct (the “Model Muddle”) • A lot of confusion results from lumping them in together and making generic statements about them as a collective (e.g. the theoretical virtues) • From a philosophical perspective, an importance difference is that they can have different purposes • They can: predict, explain, illustrate, explore consequences, communicate, be an analogy etc. • Being good at one purpose does not necessarily make a model good at another
  4. 4. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 4 Justifying Models • Whilst one can be as sloppy as one wants, playing with models in the privacy of one’s own home, if one is presenting it in a public forum, one should indicate how it is should be judged... • ...in other words be crystal clear about its purpose • So the audience can know how to assess its goodness for its purpose, and hence how to take on board the lessons learned • Unfortunately, many authors are not clear, maybe they are confused or simply want to impress by implying uses which have not been demonstrated
  5. 5. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 5 Analogical Uses of Models • Humans are good at using analogies – it is a powerful way of thinking • Indeed analogy may be at the root of how language works (Lackoff 1987) • Almost anything can be used as an analogy... including models! • A key property of analogies is that the relationship from model to modelled is not precise, but flexibly inferred by the audience each time • In particular, the relationship with data is not specified but left to the imagination
  6. 6. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 6 Empirical Uses of Models • These are models where there is a precise and direct relationship with some data • This includes models whose purpose is prediction, but also those with explanatory and descriptive purposes • In theory, the empirical fit to the data could be checked by a machine • Here, the model itself encodes knowledge (given how it is set up etc.)… • …although it may be that it also suggests further human understanding or has other uses…
  7. 7. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 7 Intuitive understanding expressed in normal language Observations of the natural system of concern Common-SenseComparison “Common-Sense” Understanding
  8. 8. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 8 Intuitive understanding expressed in normal language Observations of the natural system of concern Data obtained by measuring the system Models of the processes in the system EmpiricalComparisons Using Empirical Models
  9. 9. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 9 Intuitive understanding expressed in normal language Observations of the natural system of concern Models of the processes in the system Common-SenseComparison Using models analogically No precise and direct relationship
  10. 10. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 10 The Limitations of a Purely Anthropomorphic Epistemology • Just because what matters to us personally (as humans) is our mental understanding, does not mean that all knowledge comes down to this • In particular, what is interesting to us, what is simple, what is comprehensible to us etc. is not a good guide to what is true or even as a guide to what a good empirical model might be like • This will become increasingly apparent as machines (AIs) encode and act upon knowledge that is so complicated that no human completely understands it
  11. 11. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 11 The Advantage of Formal Models • Not that they are particularly suited to representing observed phenomena (e.g. social, economic, biological…) • But due to the fact that they can be unambiguously transmitted – i.e. without re-interpretation by the recipient • This allows for a social way of working, where a community of researchers can check, critique, develop and use the formal models • If any set of models undergoes extensive copying, some variation and selection you get a form of evolution (though like biological evolution this does not necessarily imply a direction or progress)
  12. 12. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 12 E.G.: Models in Quantum Mechanics • Mathematical (and derived computational) models in quantum mechanics have been incredibly successful empirically • Although the models have developed, the underlying mathematics (Schrödinger's wave equation) has remained the same • The analogical models used to get some human understanding have changed according to long- range fashions (Copenhagen, many-worlds etc.) • The analogical models are important to help direct research, but the empirically important core is the empirical models
  13. 13. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 13 How do we want to select for good models? How we select for model (the way we determine which models get preferentially copied in the future) determines the natures of the outcomes of the evolutionary process (given some assumptions): • If we select models by their empirical success then we might end up with complicated but empirically good models • If we select models by how interesting they are (to us humans) then we might end up with relatively simple and interesting set, but there is go reason to suppose they are empirically good
  14. 14. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 14 The Limitations of Analogical Models • Due to our unconscious ability of understanding analogies, analogical models can give an misleading impression of its potential generality • The danger of analogical models are we take it: 1. too seriously, it is just a way of thinking about stuff 2. as a good starting point for the development of models for other purposes (e.g. empirical explanation) 3. as somehow generally true merely approximate 4. supported by seeking out confirmation
  15. 15. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 15 The Usefulness of Analogical Models • As humans, we need some way of thinking about unfamiliar or complicated stuff, analogies provide way of doing this • We do not need training in it, we are naturals • For example, as a kind of meta-knowledge – how to think about the horrible empirical models • Thus they can form a part of a chain of models each modelling/abstracting from the next, that allow us to leverage useful tasks for systems beyond our ability to understand directly • Simple models can be an effective counter- example, but only if the model is plausible!
  16. 16. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 16 Making models of models of … Hegselmann-Kruse Model epsilon-diagrams phase diagrams Narrative Model
  17. 17. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 17 Making models of models of … Data-Integration Simulation Model Micro-Evidence Macro-Data Reduced Simulation Models Analytic Model Even Simpler Simulation Model Done by social scientists Done by physicists Analogical Model
  18. 18. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 18 Will ABMs evolve empirically? This is unclear it depends on a number of things: • Whether there is enough copying of models • Whether the variation that modeller introduce are sufficient to cover enough of the possibilities • How models are selected for success: empirically? for interest? for simplicity? How we do science matters! – A useful outcome is not guaranteed by just doing what is interesting or what we can (directly) understand
  19. 19. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 19 Summary • Just as machines extend our physical abilities, models extend our mental abilities • But they are not all of a kind, they have different purposes: theoretical exposition, empirical explanation, prediction, description etc. • Each purpose will entail different theoretical virtues, e.g. simplicity for illustration, not necessarily for empirical explanation etc. • The virtues are not just a matter of personal choice • Care needs to be taken not to conflate models established for different purposes
  20. 20. The Possible Evolution of Empirical ABMs, Bruce Edmonds, ABMs in Philosophy, Bochum, March. 2019. slide 20 The End Bruce Edmonds http://bruce.edmonds.name Centre for Policy Modelling http://cfpm.org A version of these slides are available at: http://slideshare.net/BruceEdmonds @

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