Queueing Lessons Dr. Ron Lembke Operations Management.

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Presentation transcript:

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. You don’t get stuck behind a slow person

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

Multiple Servers, One Line

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

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

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

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?

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

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

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