Sunday, October 25, 2020

Agent-Based Modeling in ExtendSim


Is it possible to build an agent-based simulation (ABS) model in ExtendSim? Certainly. We recently helped build a model of the intensive care unit (ICU) of a hospital. The purpose of the model was to study the impact of facility design. We used ExtendSim to compare two facility designs by looking at how nurses interact with each other when requesting patient care assistance. The model turned out to a good example of an ABS model, and it was straightforward to build in ExtendSim. 

Typical discrete event simulation (DES) models are process-based where entities flow through a defined set process. ABS models are a special case of DES models. ABS models use autonomous decision-making entities called agents where the agents make decisions based on a set of rules. In typical ABS models, the focus is on the agent behaviors.

The ICU model turned out to be a state diagram similar to the one shown below (please note that I have simplified the state diagram for illustrative purposes). The nurses are modeled as agents in the system. The nurse behavior is modeled using their states and the rules around why the nurse agents transition from one state to another. 

Sometimes, a nurse’s patient requests help, so the nurse agent transitions from idle to helping the patient. Other times, a nurse who is with the patient, requests help from another nurse that is currently idle. This forces a state change in two nurse agents. One nurse agent transitions from “I am helping my patient” to “another nurse is helping me.” Meanwhile, the nurse agent who is helping will transition from “idle” to “I am helping another nurse.” When the helping task is complete, both nurse agents transition back to their idle states and on with their next task.

This example is simple to build in ExtendSim once you understand the modeling components involved. This includes the ExtendSim database, a few advanced database features of which the link alert is the most important, and the Queue Equation block (all of these topics are covered in the Advanced ExtendSim course by the way).  What we discovered from this experience is that ExtendSim is a great tool to solve certain kinds of ABS models.

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