Operations Management Waiting Lines

2 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Understanding the phenomenon of waiting  Measures of waiting-line systems Waiting time, number of waiting orders  Impact of variability/uncertainty & utilization rate  Risk pooling effect in waiting line Objectives

3 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  The Psychology of Waiting Lines About experience of waiting Actual waiting time versus waiting time that feels like  Laws of service Satisfaction = Perception – Expectation It is hard to play catch-up ball The Article

4 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Unoccupied time feels longer than occupied time  Pre-process waits feels longer than in-process waits  Anxiety makes waits seem longer  Uncertain waits are longer than known, finite waits  Unexplained waits are longer than explained waits  Unfair waits are longer than equitable waits  The more valuable the service, the longer I will wait  Solo waiting feels longer than group waiting Principals of Waiting

5 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  The time of the arrival of an order is not known ahead of time The time a telephone call is made is random  The service time is not known ahead of time The time a customers spends on the web page of Amazon.com is random The time a customer spends speaking with the teller in the bank is unknown Characteristics of Queuing Systems

6 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  This leads to : Idleness of resources Waiting time of customers (orders) to be processed  We are interested in evaluating: Average waiting time in the queue and in the system The average number of orders (customers) waiting to be processed  Waiting time and average number are measures Characteristics of Queuing Systems

7 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  This leads to : Idleness of resources Waiting time of customers (orders) to be processed  We are interested in evaluating: Average waiting time in the queue and in the system The average number of orders (customers) waiting to be processed  Waiting time and average number are measures Characteristics of Queuing Systems

8 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Questions: Can we process the orders? How many orders will wait in the queue? How long will orders wait in the queue? What is the utilization rate of the facility? A Deterministic System: Example 1

9 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 A Deterministic System: Example 1

10 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 A Deterministic System: Example 1

11 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Arrival rate = 1/10 per minutes  Processing rate = time 1/9 per minute  Utilization – AR/PR = (1/10)/(1/9) = 0.9 or 90%  On average 0.9 person is in the system Utilization

12 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 A Deterministic System: Example 1 Utilization:90% Variability:0.00 Average Throughput time:9.00minutes Average Wait in Queue:0.00minutes Average Number in system:0.90jobs

13 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  What if arrivals are not exactly every 10 minutes?  Let’s open the spreadsheet. Known but Uneven Demand: Example 2

14 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 A Deterministic System: Example 2

15 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 A Deterministic System: Example 2 Arrival TimeService Time Interarrival time Throughput timeDeparture Waiting time in Queue

16 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 A Deterministic System: Example 2

17 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 Observations: 1. Utilization is below 100% (machine is idle 14% of the time). 2. There are 1.12 orders (on average) waiting to be processed. A Deterministic System: Example 2 Average Interarrival time10.000minutesUtilization86% Average Service time9.000minutes Average Throughput Time11.70minutes Std Service time0.000minutes Average Wait in Queue2.70minutes Thoughput rate0.096 jobs / min Average # in the system1.12jobs Capacity (Service rate)0.111 jobs / min

18 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Why do we have idleness (low utilization) and at the same time orders are waiting to be processed?  Answer: Variability A Deterministic System: Example 2

19 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  How to measure variability?  Coefficient of variation: CV = Standard Deviation / Mean Known but Uneven Demand: Example 2

20 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  The interarrival time is either 5 periods with probability 0.5 or 15 periods with probability 0.5 Notice that the mean interarrival time is 10. (mean interarrival = 0.5 * * 5 = 10)  The service time is 9 periods (with certainty).  The only difference between example 3 and 1 is that the interarrival times are random. Uncertain Demand (Interarrival times): Example 3

21 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 Simulation of Uncertain Demand (Inter-arrival times): Example 3 ArrivalStartFinishWaitingIdleness

22 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 (Recall that in Example 1, no job needed to wait.) Uncertain Demand (Interarrival times): Example 3 Average Interarrival time10.200minutes Average Througput time18.98 Average Service time9.000minutes Average wait in queue9.98 Std Service time0.000minutesAverage # in queue0.98 Thoughput rate0.100 jobs / min Average in the system Capacity (Service rate)0.111 jobs / min

23 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Suppose we change the previous example and assume: Inter-arrival time170.5 probability Inter-arrival time 30.5 probability Average inter-arrival times as before 10 min. Uncertain Demand (Inter-arrival times): Example 3

24 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1 The effect of variability: higher variability in inter-arrival times results in higher average # in queue. Uncertain Demand (Interarrival times): Example 3 Average Interarrival time10.200minutes Average Througput time27.94 Average Service time9.000minutes Average wait in queue18.94 Std Service time0.000minutesAverage # in queue1.86 Thoughput rate0.100 jobs / min Average in the system Capacity (Service rate)0.111 jobs / min

25 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Can we manage demand?  What are other sources of variability/uncertainty? Can we reduce demand variability/ uncertainty?

26 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  Up to now, our service time is exactly 9 minutes.  What will happen to waiting-line and waiting-time if we have a short service time (i.e., we have a lower utilization rate)?  What will happen if our service time is longer than 10 minutes? Uncertain Demand (Inter-arrival times)

27 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  The factors that determine the performance of the waiting lines: Variability Utilization rate Risk pooling effect Key Concepts and Issues

28 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  In general, if the variability, or the uncertainty, of the demand (arrival) or service process is large, the queue length and the waiting time are also large. Rule 1

29 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  As the utilization increases the waiting time and the number of orders in the queue increases exponentially. Rule 2

30 Ardavan Asef-Vaziri Sep-09Operations Management: Waiting Lines1  In general, pooling the demand (customers) into one common line improves the performance of the system. Rule 3