What is Simulation? Chapter 1 Subset Only Last revision June 7, 2003

What is Simulation? Chapter 1 Subset Only Last revision June 7, 2003 Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 1 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 2 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 3 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 4 of 23

Simulation by Hand: The Buffon Needle Problem Estimate (George Louis Leclerc, c. 1733) Toss needle of length l onto table with stripes d ( l) apart P (needle crosses a line) Repeat; tally proportion of times a line is crossed Estimate by Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 5 of 23

Why Toss Needles? Buffon needle problem seems silly now, but it has important simulation features: Experiment to estimate something hard to compute exactly (in 1733) Randomness, so estimate will not be exact; estimate the error in the estimate Replication (the more the better) to reduce error Sequential sampling to control error — keep tossing until probable error in estimate is “small enough” Variance reduction (Buffon Cross) Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 6 of 23

Using Computers to Simulate (cont’d.) Simulation languages GPSS, SIMSCRIPT, SLAM, SIMAN (on which Arena is based, and is included in Arena) Popular, still in use Learning curve for features, effective use, syntax High-level simulators Very easy, graphical interface Domain-restricted (manufacturing, communications) Limited flexibility — model validity? Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 7 of 23

Where Arena Fits In Hierarchical structure Multiple levels of modeling Can mix different modeling levels together in the same model Often, start high then go lower as needed Get ease-of-use advantage of simulators without sacrificing modeling flexibility Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 8 of 23

When Simulations are Used Uses of simulation have evolved with hardware, software The early years (1950s-1960s) Very expensive, specialized tool to use Required big computers, special training Mostly in FORTRAN (or even Assembler) Processing cost as high as 1000/hour for a sub-286 level machine Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 9 of 23

When Simulations are Used (cont’d.) The formative years (1970s-early 1980s) Computers got faster, cheaper Value of simulation more widely recognized Simulation software improved, but they were still languages to be learned, typed, batch processed Often used to clean up “disasters” in auto, aerospace industries – – – – Car plant; heavy demand for certain model Line underperforming Simulated, problem identified But demand had dried up — simulation was too late Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 10 of 23

When Simulations are Used (cont’d.) The recent past (late 1980s-1990s) Microcomputer power Software expanded into GUIs, animation Wider acceptance across more areas – – – – Traditional manufacturing applications Services Health care “Business processes” Still mostly in large firms Often a simulation is part of the “specs” Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 11 of 23

When Simulations are Used (cont’d.) The present Proliferating into smaller firms Becoming a standard tool Being used earlier in design phase Real-time control The future Exploiting interoperability of operating systems Specialized “templates” for industries, firms Automated statistical design, analysis Networked sharing of data in real time Integration with other applications Distributed model building, execution Simulation with Arena, 3rd ed. Chapter 1 – What Is Slide 12 of 23