W07 Variability Basics - Variability Basics God does not...

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R.B. Chinnam @ WSU God does not play dice with the universe. Albert Einstein Stop telling God what to do. Niels Bohr IE7315: Production Systems (Source: Hopp & Spearman 2008) Variability Basics
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R.B. Chinnam @ WSU 2 IE7315: Production Systems (Source: Hopp & Spearman 2008) Variability Makes a Difference! Variability! Little’s Law: TH = WIP/CT, so same throughput can be obtained with large WIP, long CT or small WIP, short CT. What is the difference?
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R.B. Chinnam @ WSU Definition : Variability is anything that causes the system to depart from regular, predictable behavior Sources of Variability: 3 IE7315: Production Systems (Source: Hopp & Spearman 2008) Variability Natural/Intrinsic Variability Intrinsic Unknown and Unidentified Sources Differential skill levels Variability from Preemptive Outages (offer less control) Machine failures Power outages Material shortages Operator unavailability Variability from Nonpreemptive Outages (offer more control) Preventive Maintenance Process Changeovers Operator Breaks, Meetings, and Shift Changes Variability from Yield Loss & Rework Variability due to Flow Arrival Rates Arrival Types (single or batched) All this leads to Queuing! Unscheduled Scheduled
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R.B. Chinnam @ WSU Definition : variability without explicitly analyzed cause Sources : operator pace material fluctuations product type (if not explicitly considered) product quality Observation : natural process variability is usually in the low variability category 4 IE7315: Production Systems (Source: Hopp & Spearman 2008) Natural Variability
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R.B. Chinnam @ WSU 5 IE7315: Production Systems (Source: Hopp & Spearman 2008) Natural/Intrinsic Variability + Preemptive Outages + Nonpreemptive Outages + Yield Loss & Rework + Flow Variability 0 t 0 ( ) f t Note: Both location and dispersion are adversely affected Skewness and Kurtosis can be affected as well but are less significant Note: f(t) can follow any distribution e t ( ) e f t e t ( ) e f t e t ( ) e f t e t ( ) e f t Corrupting Influence of Variability
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R.B. Chinnam @ WSU 6 IE7315: Production Systems (Source: Hopp & Spearman 2008) Measuring Process Variability m ean process tim e of a job stan dard deviation of process tim e coefficien t of variation , C V e e e e e t σ c t Note: We often use “squared coefficient of variation” (SCV), c e 2
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R.B. Chinnam @ WSU 7 IE7315: Production Systems (Source: Hopp & Spearman 2008) Variability Classes in Factory Physics Effective Process Times: actual process times are generally LV effective process times include setups, failure outages, etc. HV, LV, and MV are all possible in effective process times Relation to Performance Cases: For balanced systems MV Practical Worst Case ( c e = 1) LV between Best Case and Practical Worst Case HV between Practical Worst Case and Worst Case 0.75 High variability (HV) Moderate variability (MV) Low variability (LV) 0 1.33 c e
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R.B. Chinnam @ WSU 8 IE7315: Production Systems (Source: Hopp & Spearman 2008) Measuring Process Variability Example T ria l M a ch in e 1
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