Operation management 9e stevenson mcgrwhill chap018

18 Management of Waiting Lines McGraw-Hill/Irwin Copyright © 2007 by The McGraw-Hill Companies, Inc All Learning Objectives      Explain why waiting lines form in systems that are underloaded Identify the goal of queuing (waiting-line) management List the measures of system performance that are used in queuing Discuss the assumptions of the basic queuing models presented Solve typical problems 18-2 Disney World  Waiting in lines does not add enjoyment  Waiting in lines does not generate revenue Waiting lines are non-value added occurrences 18-3 Waiting Lines  Queuing theory: Mathematical approach to the analysis of waiting lines  Goal of queuing analysis is to minimize the sum of two costs   Customer waiting costs  Service capacity costs Waiting lines are non-value added occurrences 18-4 Implications of Waiting Lines  Cost to provide waiting space  Loss of business  Customers leaving  Customers refusing to wait  Loss of goodwill  Reduction in customer satisfaction  Congestion may disrupt other business operations 18-5 Queuing Analysis Figure 18.1 Cost Total cost = Customer waiting cost + Total cost Capacity cost Cost of service capacity Cost of customers waiting Service capacity Optimum 18-6 System Characteristics  Population Source  Infinite source: customer arrivals are unrestricted  Finite source: number of potential customers is limited  Number of observers (channels)  Arrival and service patterns  Queue discipline (order of service) 18-7 Elements of Queuing System Figure 18.2 Processing order Arrivals Waiting line Service Exit System 18-8 Figure 18.3 Queuing Systems Multiple channel Channel: A server in a service system Multiple phase 18-9 Poisson Distribution Figure 18.4 18-10 Waiting line Models  Patient  Customers enter the waiting line and remain until served  Reneging  Waiting customers grow impatient and leave the line  Jockeying  Customers may switch to another line  Balking  Upon arriving, decide the line is too long and decide not to enter the line 18-11 Waiting Time vs Utilization Average number on time waiting in line Figure 18.7 System Utilization 100% 18-12 System Performance Measured by: Average number of customers waiting Average time customers wait System utilization Implied cost Probability that an arrival will have to wait 18-13 Queuing Models: Infinite-Source 1.Single channel, exponential service time 2.Single channel, constant service time 3.Multiple channel, exponential service time 4.Multiple priority service, exponential service time 18-14 Priority Model Processing order Arrivals Arrivals are assigned a priority as they arrive 1 Waiting line Service Exit System 18-15 Table 18.6 Finite-Source Formulas Service factor Average number waiting Average waiting time Average number running Average number being served Number in population T X= T +U L = N (1 − F ) L( T + U ) T (1 − F ) W= = N−L XF J = NF (1 − X ) H = FNX N = J + L+H 18-16 Finite-Source Queuing Not waiting or being served Waiting Being served J L H U W T J+H F= J + L+H 18-17 Other Approaches  Reduce perceived waiting time  Magazines in waiting rooms  Radio/television  In-flight movies  Filling out forms  Derive benefits from waiting  Place impulse items near checkout  Advertise other goods/services 18-18 ... waiting lines form in systems that are underloaded Identify the goal of queuing (waiting-line) management List the measures of system performance that are used in queuing Discuss the assumptions...  Loss of goodwill  Reduction in customer satisfaction  Congestion may disrupt other business operations 18-5 Queuing Analysis Figure 18.1 Cost Total cost = Customer waiting cost + Total cost