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Waiting Line Management Chapter #12
Waiting Line See Page # 291 (Book-1) One or more ‘customers’ waiting for a service. ‘Customer’ can be: › People e. g. A person waiting in line to deposit cash in a bank. › Objects e. g. A machine waiting for maintenance, Inventory waiting to be delivered. Truck waiting to be loaded etc.
Why waiting lines form? Temporary imbalance between demand capacity. Larger arrival rate than servicing rate Randomness/Variability Customers usually arrive at random intervals Variability in order lengths – some orders take longer than others
Effects of Waiting Line Waiting lines are non-value added occurrences. Waiting in lines does not add enjoyment for customers. Waiting in lines does not generate revenue for company. Costly to provide additional waiting space. Possible loss of business. › Customers refusing to wait › Customers leaving Loss of customer goodwill. Reduction in customer satisfaction. Congestion may disrupt other business operations.
Objectives of Waiting Line Analysis To improve system Utilization To minimize the sum of two costs › Customer waiting costs › Service capacity costs
Structure of Waiting-Line Problems 1. An input, or customer population, that generates potential customers. 2. A waiting line of customers. (Customers Behavior) 3. The service facility, consisting of a person (or crew), a machine (or group of machines), or both necessary to perform the service for the customer. 4. A priority rule, which selects the next customer to be served by the service facility.
Customer population Service system Waiting line Priority rule Served customers Service facilities Figure C. 1 – Basic Elements of Waiting-Line Models
Customer Population or Input Population Source Example: Number of machines needing repair when a company only has three machines. Customers from a finite source reduce the chance of new arrivals Example: The number of people who could wait in a line for gasoline. Customers from an infinite source do not affect the probability of another arrival
Customer Behavior l Customers are patient or impatient u Patient customers wait until served u Impatient customer behave in different ways: u Balking: When customer decides not to enter in line. u Jockeying: When customer switches to another line. u Reneging: When customer quits waiting and leaves the line.
The Service System Service rate depends on the structure of service system and facility. Structure of a service system depends on various factors such as: › Service time for customer › No. of lines › No. of service channels › No. of service phases
Service time for customer Service Times Constant Example: Items coming down an automated assembly line. Variable Example: People spending time shopping.
No. of lines in system › A single-line keeps servers uniformly busy and levels waiting times among customers › A multiple-line arrangement is favored when servers provide a limited set of services Service facilities (a) Single line (b) Multiple lines
Service system Arrangement u. Single-channel, single-phase u. Single-channel, multiple-phase u. Multiple-channel, single-phase u. Multiple-channel, multiple-phase u. Mixed arrangement
Service facility (a) Single channel, single phase Service facility 1 (b) Single channel, multiple phase Examples of Service Facility Arrangements Service facility 2
Service facility 1 Service facility 2 (c) Multiple channel, single phase Service facility 1 Service facility 3 Service facility 2 Service facility 4 (d) Multiple channel, multiple phase
(e) Mixed arrangement Service facility 1 Service facility 3 Routing for Service facility 2 Service facility 4 : 1– 2– 4 : 2– 4– 3 : 3– 2– 1– 4
System Arrangement examples Single Phase Single Channel Multichannel Multiphase
Priority Rule for waiting line First-come, first-served (FCFS)—used by most service systems Earlier Due Date (EDD) Shortest Processing Time (SPT) Preemptive discipline—allows a higher priority customer to interrupt the service of another customer or be served ahead of another.
Important Measures of Waiting line Analysis System Utilization Average Number of Customers Waiting Average Customer Time in System › Waiting time + processing time Average Customer Waiting Time › Typically, you don’t want to keep the customer waiting for an unreasonable amount of time Customer Waiting Costs Service Costs Probability of Lost Sales › Would like to minimize
Decision Areas for Management to avoid waiting lines 1. 2. 3. 4. 5. 6. 7. Arrival rates Line arrangement Number of service facilities Number of phases Number of servers per facility Server efficiency Priority rule
Strategies for Waiting line management Reduce perceived waiting time › › › Tell customers how long their wait will be Magazines in waiting rooms Music/television In-flight movies Filling out forms Derive benefits from waiting › Place impulse items in service facility › Advertise other goods/services › Encourage customers to come during the slack periods.