What is Simulation Chapter 1 Last revision June 7 2003 Simulation with Arena 3rd ed Chapter 1 What Is Slide 1 of 23 Simulation Is Simulation very broad term methods and applications to imitate or mimic real systems usually via computer Applies in many fields and industries Very popular and powerful method Book covers simulation in general and the Arena simulation software in particular This chapter general ideas terminology examples of applications good bad things kinds of simulation software options how when simulation is used Simulation with Arena 3rd ed Chapter 1 What Is Slide 2 of 23 Systems System facility or process actual or planned Examples abound Manufacturing facility Bank operation Airport operations passengers security planes crews baggage Transportation logistics distribution operation Hospital facilities emergency room operating room admissions Computer network Freeway system Business process insurance office Criminal justice system Chemical plant Fast food restaurant Supermarket Theme park Emergency response system Simulation with Arena 3rd ed Chapter 1 What Is Slide 3 of 23 Work With the System Study the system measure improve design control Maybe just play with the actual system Advantage unquestionably looking at the right thing But it s often impossible to do so in reality with the actual system System doesn t exist Would be disruptive expensive or dangerous Simulation with Arena 3rd ed Chapter 1 What Is Slide 4 of 23 Models Model set of assumptions approximations about how the system works Study the model instead of the real system usually much easier faster cheaper safer Can try wide ranging ideas with the model Make your mistakes on the computer where they don t count rather than for real where they do count Often just building the model is instructive regardless of results Model validity any kind of model not just simulation Care in building to mimic reality faithfully Level of detail Get same conclusions from the model as you would from system More in Chapter 13 Simulation with Arena 3rd ed Chapter 1 What Is Slide 5 of 23 Types of Models Physical iconic models Tabletop material handling models Mock ups of fast food restaurants Flight simulators Logical mathematical models Approximations and assumptions about a system s operation Often represented via computer program in appropriate software Exercise the program to try things get results learn about model behavior Simulation with Arena 3rd ed Chapter 1 What Is Slide 6 of 23 Studying Logical Models If model is simple enough use traditional mathematical analysis get exact results lots of insight into model Queueing theory Differential equations Linear programming But complex systems can seldom be validly represented by a simple analytic model Danger of over simplifying assumptions model validity Type III error working on the wrong problem Often a complex system requires a complex model and analytical methods don t apply what to do Simulation with Arena 3rd ed Chapter 1 What Is Slide 7 of 23 Computer Simulation Broadly interpreted computer simulation refers to methods for studying a wide variety of models of systems Numerically evaluate on a computer Use software to imitate the system s operations and characteristics often over time Can be used to study simple models but should not use it if an analytical solution is available Real power of simulation is in studying complex models Simulation can tolerate complex models since we don t even aspire to an analytical solution Simulation with Arena 3rd ed Chapter 1 What Is Slide 8 of 23 Popularity of Simulation Consistently ranked as the most useful popular tool in the broader area of operations research management science 1978 M S graduates of CWRU O R Department after graduation 1 Statistical analysis 2 Forecasting 3 Systems Analysis 4 Information systems 5 Simulation 1979 Survey 137 large firms which methods used 1 Statistical analysis 93 used it 2 Simulation 84 3 Followed by LP PERT CPM inventory theory NLP Simulation with Arena 3rd ed Chapter 1 What Is Slide 9 of 23 Popularity of Simulation cont d 1980 A IIE O R division members First in utility and interest simulation First in familiarity LP simulation was second 1983 1989 1993 Longitudinal study of corporate practice 1 Statistical analysis 2 Simulation 1989 Survey of surveys Heavy use of simulation consistently reported Simulation with Arena 3rd ed Chapter 1 What Is Slide 10 of 23 Advantages of Simulation Flexibility to model things as they are even if messy and complicated Avoid looking where the light is a morality play You re walking along in the dark and see someone on hands and knees searching the ground under a street light You What s wrong Can I help you Other person I dropped my car keys and can t find them You Oh so you dropped them around here huh Other person No I dropped them over there Points into the darkness You Then why are you looking here Other person Because this is where the light is Allows uncertainty nonstationarity in modeling The only thing that s for sure nothing is for sure Danger of ignoring system variability Model validity Simulation with Arena 3rd ed Chapter 1 What Is Slide 11 of 23 Advantages of Simulation cont d Advances in computing cost ratios Estimated that 75 of computing power is used for various kinds of simulations Dedicated machines e g real time shop floor control Advances in simulation software Far easier to use GUIs No longer as restrictive in modeling constructs hierarchical down to C Statistical design analysis capabilities Simulation with Arena 3rd ed Chapter 1 What Is Slide 12 of 23 The Bad News Don t get exact answers only approximations estimates Also true of many other modern methods Can bound errors by machine roundoff Get random output RIRO from stochastic simulations Statistical design analysis of simulation experiments Exploit noise control replicability sequential sampling variance reduction techniques Catch standard statistical methods seldom work Simulation with Arena 3rd ed Chapter 1 What Is Slide 13 of 23 Different Kinds of Simulation Static vs Dynamic Continuous change vs Discrete change Can the state change continuously or only at discrete points in time Deterministic vs Stochastic Does time have a role in the model Is everything for sure or is there uncertainty Most operational models Dynamic Discrete change Stochastic Though Chapter 11 discusses continuous and combined discretecontinuous models Simulation with Arena 3rd ed Chapter 1 What Is