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Queueing Lessons

Queueing Lessons. Dr. Ron Lembke Operations Management. Which is better, one line or two?. One line: Airport check-in, security, Best Buy (holidays), Michaels, Taco Bell, most banks Multiple lines: Grocery stores, Wal-Mart In the past: banks, McDonalds, etc.

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Queueing Lessons

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  1. Queueing Lessons Dr. Ron Lembke Operations Management

  2. Which is better, one line or two? • One line: • Airport check-in, security, Best Buy (holidays), Michaels, Taco Bell, most banks • Multiple lines: • Grocery stores, Wal-Mart • In the past: banks, McDonalds, etc. • You don’t get stuck behind a slow person

  3. One line or four? • This cashier is fast/slow! Should I switch? • Sometimes workload is visible: • Grocery store – food in cart • Airport - # of suitcases? • Bank – no clue at all • Starbucks – one person – drinks for office 1 2 3 4

  4. Multiple Servers, One Line 1 2 3 4

  5. Multiple Servers, One Line 1 2 3 4

  6. Separating Multiple Tasks • Then: CT = 6 min/customer = 10/hr • Place order and pay (2 min) • Cashier gets food from heat lamps (3 min) • Cashier gets your drink (1 min) • Now: CT = 2 min/customer = 30/hr • Place order and pay (2 min), get own drink • Customers/labor hour = 200% increase! • Plus, free refills! (and suicides, if you’re a middle schooler) • Guaranteed right drink: regular/diet, how much ice

  7. Cashier Gets Food and Drink? time Order/pay Food prep Get Drink If you just pay the cashier and get your own drink while someone else gets the food, it’s a lot faster

  8. System Structure • The more comlicated the system, the harder it is to model: • Separate lines • Separate tellers, etc.

  9. Factors to Consider • Arrival patterns, arrival rate • Size of arrival units – 1,2,4 at a time? • Degree of patience • Length line grows to • Number of lines – 1 is best • Does anyone get priority?

  10. Service Time Distribution • Deterministic – each person always takes 5 minutes • Random – low variability, most people take similar amounts of time • Random – high variability, large difference between slow & fast people

  11. Now what? • Simulate! • Build a computer version of it, and try it out • Tweak any parameters you want • Change it as much as you want • Try it out with zero risk

  12. What did we learn? • Queueing Theory can help with simple capacity decisions • One line is better than two • Splitting tasks can get people through faster • Reality is filled with queueing systems that are far more complex than that • Simulation needed for more complex ones

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