'El Farol Bar Problem' in Netlogo
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Modeling the ‘El Farol Bar Problem’ in NetLogo ∗ PRELIMINARY DRAFT Mark Garofalo† Dexia Bank Belgium Boulevard Pach´eco 44 B-1000 Brussels, Belgium http://www.markgarofalo.com Abstract believe most will go, nobody will go’ (Arthur (1994)). Resource limitation is characteristic to many other areas Arthur’s ‘El Farol Bar Problem’ (Arthur (1994)) is a such as road traffic, computer networks like internet or finan- metaphor of a complex economic systems’ model based on cial markets. These systems are complex in the sense that interaction and it is perfectly suited for the multi-agent sim- they composed by a large variety of elements and that the ulation framework. It is a limited resource model in which interaction in between these elements are non-linear and not agents are boundedly rational and where no solution can be completely captured (LeMoigne (1990), Kauffman (1993)). deduced a priori. In this paper we discuss in particular the Paraphrasing Casti (1996), in the EFBP the agents are in a implementation of the model in NetLogo, a multi-agent universe in which their ‘forecasts (...) act to create the world platform introduced by Wilensky (1999). Our aim is to they are trying to forecast’1. Arthur’s simple scenario pro- translate Arthur’s model described in natural language into vides useful insights into complex systems and this is why a workable computational agent-based tool. Throughout the it is viewed as paradigmatic. process of setting up the tool we discuss the necessary as- Modeling in mainstream economics is generally equation sumptions made for it’s implementation. We also conduct based. However to study the EFBP, rather than considering simulations to illustrate the model and to show that the de- a particular stochastic process, a computational agent based sign of the tool is appropriate. approach seems to be an appropriate alternative2 (Gilbert Keywords: Complex system, El Farol Bar Problem, model, and Troitzsch (1999)). A computational agent-based model multi-agent, NetLogo, simulation. includes interacting agents who rely on their experience, rules and information to determine their actions. Agents can 1 Introduction interact in both space and time, creating emerging dynamic patterns, and potentially new behaviours not introduced by The problem set up by Arthur in the ‘El Farol Bar Prob- the modeler (see Reynolds (1987)). Computational models lem’, here after EFBP, is a popular paradigm of complex are not constrained by the limits imposed by the desire of economic systems in which agents self-organise themselves elegance in mathematics. The models also permit hetero- while they are in competition for a limited resource, without geneity in not only agent preferences but also their behav- possibility of communication, thus there is no solution de- ior, offering flexibility. Several references discuss multi-agent ductible a priori. In this problem, every agent has to choose systems; Weiss (2000) and Ferber (1995) are good ones. to go or not to go to the ‘El Farol’ bar each week using a The agent-based approach seems therefore to be a very predictor of the next attendance. It is given that the agents natural and flexible way to model the EFBP, and that is try to avoid crowd and they will not show up at the bar what we have decided to apply here. The EFBP has al- if they forecast that more than sixty percent of the agents ready been studied by simulation in Arthur’s paper and go. But since there is no single predictor that can work for by Fogel, Chellapilla, and Angeline (1999). Mathemati- everybody at the same time, there is no deductively ratio- cal formalisation has also been used by Challet, Marsili, nal solution. In fact the agents would have to know which and Ottino (2004) based on the findings from the minor- predictors the others will use, which they don’t, so from the ity games (Challet (2000)), however we propose here to point of view of the agent, the problem is ill-defined. They rewrite step by step an agent-based version of the model and can only observe the attendance, thus their reasoning, as we perform simulations of the collective behaviour to align pointed out by Arthur, is inductive. The consequence is a them with Arthur’s findings. Rewriting models that others self-defeating prophecy feature because at each attendance have described is common practice in the multi-agent based forecast, ‘if all believe few will go, all will go (...) and if all simulation community in order to understand them more deeply and reproduce the stated results (Hales, Rouchier, ∗Copyright Mark Garofalo, 2006. Please do not copy or reprint and Edmonds (2003), Axelrod (1997)). without permission from the author. This version May 2, 2006. Up- dates are available on my website. Comments are welcome. Email The rest of this paper is organised as follows. Section contact [email protected] 2 presents the EFBP and discusses Arthur’s modeling as- †Any opinions expressed here reflect the current judgment of the sumptions. Section 3 presents NetLogo, the multi-agent author, and do not necessarily reflect the opinion of Dexia Bank Bel- gium or any of its subsidiaries and affiliates. Neither Dexia Bank 1I took this quote from Zambrano (2004). Belgium nor its subsidiaries/affiliates accept any responsibility for 2For a deeper discussion on equation based modeling compared to liabilities arising from use of this document or its contents. agent based modeling, see Parunak, Savit, and Riolo (1998). 1 platform we use to perform the simulations and presents At each time step, week after week, the agent uses the the set up of the user interface. Section 4 briefly discusses most performing predictor - currently the most accurate - the implementation of the EFBP. In order to validate the over the past d weeks, to decide to go or not to the bar, called computational tool, we perform simulations on the collective the active predictor. Thus if the attendance prediction for ap bahaviour and discuss the results in Section 5 and finally period t given history ηt of the active predictor s (A(ηt) Section 6 concludes. is greater than L the action for the ith agent based on that i ap information, a [s (A(ηt))], is 0 and 1 otherwise. The way 2 Brian Arthur’s Model in which Arthur attributes the performance to the predictors in order to classify them to determine the active predictor is El Farol is a bar at Santa Fe, New Mexico, where a band not specified, however reviewing literature, we propose three plays Irish music on Thursday nights, and, like in many interpretations. places, the bar is enjoyable only if it is not too crowded. We start with what we call the absolute precision. This The N (indexed by i) inhabitants of Santa Fe have the same interpretation establishes a function rewarding the predic- fixed preferences L (the open level), or more generally l, tor indicating the best the opening or not of the bar, that with l = L/N; the question asked by Arthur is to find out means the occurrence of the event. Say for example, with how these N agents will select the following binary action an attendance of A(t) = 55, a predictor giving 10 or 50 is ai ∈ {0, 1}, where 1 is go to the bar and try to avoid crowd equivalent. Parkes and Steinig (1997) use this interpretation to listen to good music, and 0 is not to go, and this without for the precision of the predictors of the inhabitants, and is any communication or exogeneous information. Thus the measured by associating a confidence level. They compute total attendance is A = P ai and if A > L, the bar does it with the ‘percentage of times that the predictor has been not open, otherwise the bar opens and they have a great on the correct side of sixty percent’. The inhabitant, they evening (A ≤ L). continue, will simply use at each time step the predictor In order to take their decision, the only available piece of with the highest confidence level. information to predict the opening at the week t, A(t), that The second interpretation is one we call the relative pre- the inhabitants have is the attendance from the d previous cision, and has been used by Brandouy (2003), who gives weeks3, an interpretation of the efficiency of the predictors by its ‘difference between the attendance it has just predicted and ηt = {A(t − d), ..., A(t − 2),A(t − 1)} , (1) the realized attendance once every agent has decided to go or not to the bar’, (implicitly supposed in absolute value). The quality of this evaluation function is to be an atten- published each Friday morning in the local paper. dance measure, however, on the contrary to the previous On this basis, they formulate forecasts of A(t) using mod- one, the past performance is not taken into account. els called predictors. Arthur supposed that there exists a A paper from Zambrano (2004) seems however to take large amount (several dozen), n, of variations of predictors, away all doubts of this discussion by stating that his eval- j j 1 2 n called the ‘alphabetic soup’, S , with S = s , s , ..., s uation function has been given to him in Arthur’s original from which each agent N selects randomly a portfolio k from code thanks to Bruce Edmonds, which we call the ‘original’ S, with k < n. A predictor forecasting for example an at- precision, and is given as follows tendance lower than L implies that the agents having that predictor in their portfolio, and using that one to forecast, j j j will go to the bar because he expects that it will not be Ut(s ) = λUt−1(s ) + (1 − λ) | s (A(ηt)) − A(t) |, (2) crowded.