Service Engineering (Science, Management)
Avi Mandelbaum, HKUST
September 2011
Course Contents
• Introduction to “Services” and “Service-Engineering”
• The Two Prerequisites: Measurements, Models (Operational)
• Empirical (Data-Based) Models
• Fluid (Deterministic) Models
• Stochastic Framework: Dynamic-Stochastic PERT/CPM
• The Building Blocks of a Basic Service Station:
– Arrivals; Forecasting
– Service Durations; Workload
– (Im)Patience; Abandonment
– Returns (During, After; Positive, Negative)
• Stochatic Models of a Service Station
– Markovian Queues: Erlang B/C/A,. . .,/R, Jackson
– Non-Parametric Queues: G/G/n, ...
• Operational Regimes and Staffing: ED, QD, QED
• Heterogeneous Customers and Servers (CRM, SBR)
1
Introduction to “Services”
U.S. Employment by Sector, 1850 - 2000+
0
0.1
0.2
0.3
0.4
0.5
0.6
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
0.7
0.8
1960
1970
1980
1990
2000
% Employment
Year
Service
Manufacturing
Agriculture
We focus on:
• Function: Operations (vs./plus IT, HRM, Marketing)
• Dimension: Accessibility, Capacity (vs. RM, SCM,...)
• Modelling Framework: Queueing Theory (plus Science)
• Applications: Call/Contact Centers (Healthcare,...)
3
Scope of the Service Industry
• Wholesale and retail trade;
• Government services;
• Healthcare;
• Restaurants and food;
• Financial services;
• Transportation;
• Communication;
• Education;
• Hospitality business:
• Leisure services.
Our Application Focus: telephone call centers,
which play an important role in most of these sectors.
4
Services: Subjective Trends
”Everything is Service”
Rather than buying a product, why not buy only the service
it provides? For example, car leasing; or, why setup and run
a help-desk for technical support, with its costly fast-to-obsolete
hardware, growing-sophisticated software, high-skilled peopleware
and ever-expanding infoware, rather than let outsourcing do it
all for you?
“Data; Technology and Human Interaction
Far too little reliance on data, the language of nature, in
formulating models for the systems and processes of the
deepest importance to human beings, namely those in
which we are actors. Systems with fixed rules, such as physical
systems, are relatively simple, whereas systems involving human
beings expressing their microgoals . . . can exhibit incredible com-
plexity; there is yet the hope to devise tractable models through
remarkable collective effects . . .
(Robert Herman: ”Reflection on Vehicular Traffic Science”.)
Fusion of Disciplines: POM/IE, Marketing, IT, HRM
The highest challenge facing banks with respect to efficient and
effective innovation lies in the ”New Age Industrial Engi-
neer” that must combine technological knowledge with process
design in order to create the delivery system of the future.
(Frei, Harker and Hunter: ”Innovation in Retail Banking”).
5
Service-Engineering
Goal (Subjective):
Develop scientifically-based design principles (rules-of-thumb)
and tools (software) that support the balance of service quality,
process efficiency and business profitability, from the (often
conflicting) views of customers, servers and managers.
Contrast with the traditional and prevalent
• Service Management (U.S. Business Schools)
• Industrial Engineering (European/Japanese Engineering Schools)
Additional Sources (all with websites):
• Fraunhofer IAO (Service Engineering, 1995): ... application
of engineering science know-how to the service sector ... mod-
els, methods and tools for systematic development and design
of service products and service systems ...
• NSF SEE (Service Enterprise Engineering, 2002): ... Cus-
tomer Call/Contact Centers ... staff scheduling, dynamic pric-
ing, facilities design, and quality assurance ...
• IBM SSME (Services Science, Management and Engineering,
2005): ... new discipline brings together computer science,
operations research, industrial engineering, business strategy,
management sciences, social and cognitive sciences, and legal
sciences ...
6
Staffing: How Many Servers?
Fundamental problem in service operations: Healthcare, . . . , or
Call Centers, as a representative example:
• People: ≈ 70% operating costs; ≥ 3% U.S. workforce.
• Business-Frontiers but also Sweat-Shops of the 21st Century.
Reality
• Complex and becoming more so
• Staffing is Erlang-based (1913!)
=⇒ Solutions urgently needed
• Technology can accommodate smart protocols
• Theory lags significantly behind needs
=⇒ Ad-hoc methods prevalent: heuristics- or simulation-based.
Research Progress based on
• Simple Robust Models , for theoretical insight into
complex realities. Their analysis requires and generates:
• Data-Based Science: Model, Experiment, Validate, Refine.
• Management Principles, Tools: Service Engineering .
7
The First Prerequisite:
Data & Measurements
Robert Herman (“Father” of Transportation Science): Far too little
reliance on Data, the language of nature, in formulating
models for the systems of the deepest importance to human beings,
namely those in which we are actors.
Empirical “Axiom”: The Data One Needs is Never There
For One To Use (Always Problems with Historical Data).
Averages do NOT tell the whole story
Individual-Transaction Level Data: Time-Stamps of Events
• Face-to-Face: T, C, S, I, O, F (QIE, RFID)
• Telephone: ACD, CTI/CRM, Surveys
• Internet: Log-files
• Transportation: measuring devices on highways/intersections
Our Databases: Operations (vs. Marketing, Surveys, . . .)
• Face-to-Face data (branch banking) – recitations; QUESTA
• Telephone data (small banking call center) – homework; JASA
• DataMOCCA (large cc’s: repository, interface) – class/research;
Website
Future Research:
Healthcare, Multimedia, Field-Support; Operation+Marketing,
8
Measurements: Face-to-Face Services
23 Bar-Code Readers at an Israeli Bank
9
Measurements: Telephone ServicesLog-File of Call-by-Call Data
vru+line call_id customer_id priority type date vru_entry vru_exit vru_time q_start q_exit q_time outcome ser_start ser_exit ser_time server
AA0101 44749 27644400 2 PS 990901 11:45:33 11:45:39 6 11:45:39 11:46:58 79 AGENT 11:46:57 11:51:00 243 DORIT
AA0101 44750 12887816 1 PS 990905 14:49:00 14:49:06 6 14:49:06 14:53:00 234 AGENT 14:52:59 14:54:29 90 ROTH
AA0101 44967 58660291 2 PS 990905 14:58:42 14:58:48 6 14:58:48 15:02:31 223 AGENT 15:02:31 15:04:10 99 ROTH
AA0101 44968 0 0 NW 990905 15:10:17 15:10:26 9 15:10:26 15:13:19 173 HANG 00:00:00 00:00:00 0 NO_SERVER
AA0101 44969 63193346 2 PS 990905 15:22:07 15:22:13 6 15:22:13 15:23:21 68 AGENT 15:23:20 15:25:25 125 STEREN
AA0101 44970 0 0 NW 990905 15:31:33 15:31:47 14 00:00:00 00:00:00 0 AGENT 15:31:45 15:34:16 151 STEREN
AA0101 44971 41630443 2 PS 990905 15:37:29 15:37:34 5 15:37:34 15:38:20 46 AGENT 15:38:18 15:40:56 158 TOVA
AA0101 44972 64185333 2 PS 990905 15:44:32 15:44:37 5 15:44:37 15:47:57 200 AGENT 15:47:56 15:49:02 66 TOVA
AA0101 44973 3.06E+08 1 PS 990905 15:53:05 15:53:11 6 15:53:11 15:56:39 208 AGENT 15:56:38 15:56:47 9 MORIAH
AA0101 44974 74780917 2 NE 990905 15:59:34 15:59:40 6 15:59:40 16:02:33 173 AGENT 16:02:33 16:26:04 1411 ELI
AA0101 44975 55920755 2 PS 990905 16:07:46 16:07:51 5 16:07:51 16:08:01 10 HANG 00:00:00 00:00:00 0 NO_SERVER
AA0101 44976 0 0 NW 990905 16:11:38 16:11:48 10 16:11:48 16:11:50 2 HANG 00:00:00 00:00:00 0 NO_SERVER
AA0101 44977 33689787 2 PS 990905 16:14:27 16:14:33 6 16:14:33 16:14:54 21 HANG 00:00:00 00:00:00 0 NO_SERVER
AA0101 44978 23817067 2 PS 990905 16:19:11 16:19:17 6 16:19:17 16:19:39 22 AGENT 16:19:38 16:21:57 139 TOVA
AA0101 44764 0 0 PS 990901 15:03:26 15:03:36 10 00:00:00 00:00:00 0 AGENT 15:03:35 15:06:36 181 ZOHARI
AA0101 44765 25219700 2 PS 990901 15:14:46 15:14:51 5 15:14:51 15:15:10 19 AGENT 15:15:09 15:17:00 111 SHARON
AA0101 44766 0 0 PS 990901 15:25:48 15:26:00 12 00:00:00 00:00:00 0 AGENT 15:25:59 15:28:15 136 ANAT
AA0101 44767 58859752 2 PS 990901 15:34:57 15:35:03 6 15:35:03 15:35:14 11 AGENT 15:35:13 15:35:15 2 MORIAH
AA0101 44768 0 0 PS 990901 15:46:30 15:46:39 9 00:00:00 00:00:00 0 AGENT 15:46:38 15:51:51 313 ANAT
AA0101 44769 78191137 2 PS 990901 15:56:03 15:56:09 6 15:56:09 15:56:28 19 AGENT 15:56:28 15:59:02 154 MORIAH
AA0101 44770 0 0 PS 990901 16:14:31 16:14:46 15 00:00:00 00:00:00 0 AGENT 16:14:44 16:16:02 78 BENSION
AA0101 44771 0 0 PS 990901 16:38:59 16:39:12 13 00:00:00 00:00:00 0 AGENT 16:39:11 16:43:35 264 VICKY
AA0101 44772 0 0 PS 990901 16:51:40 16:51:50 10 00:00:00 00:00:00 0 AGENT 16:51:49 16:53:52 123 ANAT
AA0101 44773 0 0 PS 990901 17:02:19 17:02:28 9 00:00:00 00:00:00 0 AGENT 17:02:28 17:07:42 314 VICKY
AA0101 44774 32387482 1 PS 990901 17:18:18 17:18:24 6 17:18:24 17:19:01 37 AGENT 17:19:00 17:19:35 35 VICKY
AA0101 44775 0 0 PS 990901 17:38:53 17:39:05 12 00:00:00 00:00:00 0 AGENT 17:39:04 17:40:43 99 TOVA
AA0101 44776 0 0 PS 990901 17:52:59 17:53:09 10 00:00:00 00:00:00 0 AGENT 17:53:08 17:53:09 1 NO_SERVER
AA0101 44777 37635950 2 PS 990901 18:15:47 18:15:52 5 18:15:52 18:16:57 65 AGENT 18:16:56 18:18:48 112 ANAT
AA0101 44778 0 0 NE 990901 18:30:43 18:30:52 9 00:00:00 00:00:00 0 AGENT 18:30:51 18:30:54 3 MORIAH
AA0101 44779 0 0 PS 990901 18:51:47 18:52:02 15 00:00:00 00:00:00 0 AGENT 18:52:02 18:55:30 208 TOVA
AA0101 44780 0 0 PS 990901 19:19:04 19:19:17 13 00:00:00 00:00:00 0 AGENT 19:19:15 19:20:20 65 MEIR
AA0101 44781 0 0 PS 990901 19:39:19 19:39:30 11 00:00:00 00:00:00 0 AGENT 19:39:29 19:41:42 133 BENSION
AA0101 44782 0 0 NW 990901 20:08:13 20:08:25 12 00:00:00 00:00:00 0 AGENT 20:08:28 20:08:41 13 NO_SERVER
AA0101 44783 0 0 PS 990901 20:23:51 20:24:05 14 00:00:00 00:00:00 0 AGENT 20:24:04 20:24:33 29 BENSION
AA0101 44784 0 0 NW 990901 20:36:54 20:37:14 20 00:00:00 00:00:00 0 AGENT 20:37:13 20:38:07 54 BENSION
AA0101 44785 0 0 PS 990901 20:50:07 20:50:16 9 00:00:00 00:00:00 0 AGENT 20:50:15 20:51:32 77 BENSION
AA0101 44786 0 0 PS 990901 21:04:41 21:04:51 10 00:00:00 00:00:00 0 AGENT 21:04:50 21:05:59 69 TOVA
AA0101 44787 0 0 PS 990901 21:25:00 21:25:13 13 00:00:00 00:00:00 0 AGENT 21:25:13 21:28:03 170 AVI
AA0101 44788 0 0 PS 990901 21:50:40 21:50:54 14 00:00:00 00:00:00 0 AGENT 21:50:54 21:51:55 61 AVI
AA0101 44789 9103060 2 NE 990901 22:05:40 22:05:46 6 22:05:46 22:09:52 246 AGENT 22:09:51 22:13:41 230 AVI
AA0101 44790 14558621 2 PS 990901 22:24:11 22:24:17 6 22:24:17 22:26:16 119 AGENT 22:26:15 22:27:28 73 VICKY
AA0101 44791 0 0 PS 990901 22:46:27 22:46:37 10 00:00:00 00:00:00 0 AGENT 22:46:36 22:47:03 27 AVI
AA0101 44792 67158097 2 PS 990901 23:05:07 23:05:13 6 23:05:13 23:05:30 17 AGENT 23:05:29 23:06:49 80 VICKY
AA0101 44793 15317126 2 PS 990901 23:28:52 23:28:58 6 23:28:58 23:30:08 70 AGENT 23:30:07 23:35:03 296 DARMON
AA0101 44794 0 0 PS 990902 00:10:47 00:12:05 78 00:00:00 00:00:00 0 HANG 00:00:00 00:00:00 0 NO_SERVER
AA0101 44795 0 0 PS 990902 07:16:52 07:17:01 9 00:00:00 00:00:00 0 AGENT 07:17:01 07:17:44 43 ANAT
AA0101 44796 0 0 PS 990902 07:50:05 07:50:16 11 00:00:00 00:00:00 0 AGENT 07:50:16 07:53:03 167 STEREN
10
Measurements:Prevalent Averages (ACD Data)
11
Beyond Averages: Waiting Times in a Call Center
Small Israeli Bank
Time
0 30 60 90 120 150 180 210 240 270 300
29.1 %
20 %
13.4 %
8.8 %
6.9 %
5.4 %
3.9 %3.1 %
2.3 %1.7 %
Mean = 98
SD = 105
Large U.S. Bank
0
2
4
6
8
10
12
14
16
18
20
2 5 8 11 14 17 20 23 26 29 32 35
Time
Relative frequencies, %
Medium Israeli Bank
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380
Time (Resolution 1 sec.)
Relative frequencies, %
8
13
The Second Prerequisite:(Operational) Models
Empirical Models
• Conceptual
– Service-Process Data = Flow Network
– Service Networks = Queueing Networks
• Descriptive
– QC-Tools: Pareto, Gantt, Fishbone Diagrams,...
– Histograms, Hazard-Rates, ...
– Data-MOCCA: Repository + Interface
• Explanatory
– Nonparametric: Comparative Statistics, Regression,...
– Parametric: Log-Normal Services, (Doubly) Poisson Ar-
rivals, Exponential (Im)Patience
Analytical Models
• Fluid (Deterministic) Models
• Stochastic Models (Birth & Death, G/G/n, Jackson,...)
14
Conceptual Model:
Service Networks = Queueing Networks
! People, waiting for service: teller, repairman, ATM
! Telephone-calls, to be answered: busy, music, info.
! Forms, to be sent, processed, printed; for a partner
! Projects, to be developed, approved, implemented
! Justice, to be made: pre-trial, hearing, retrial
! Ships, for a pilot, berth, unloading crew
! Patients, for an ambulance, emergency room, operation
! Cars, in rush hour, for parking
! Checks, waiting to be processed, cashed
! Queues Scarce Resources, Synchronization Gaps
Costly, but here to stay
– Face-to-face Nets (Chat) (min.)
– Tele-to-tele Nets (Telephone) (sec.)
– Administrative Nets (Letter-to-Letter) (days)
– Fax, e.mail (hours)
– Face-to-ATM, Tele-to-IVR
– Mixed Networks (Contact Centers)
15
Conceptual Model:
Bank Branch = Queueing Network
23
Teller
Entrance
Tourism
Xerox
Manager
Teller
Entrance
Tourism
Xerox
Manager
Bottleneck!16
Bank Branch: A Queuing Network
Transition Frequencies Between Units in The Private and Business Sections:
Private Banking Business
To Unit Bankers Authorized Compens - Tellers Tellers Overdrafts Authorized Full Exit
From Unit Personal - ations Personal Service
Bankers 1% 1% 4% 4% 0% 0% 0% 90%
PrivateAuthorized
Personal12% 5% 4% 6% 0% 0% 0% 73%
Banking Compensations 7% 4% 18% 6% 0% 0% 1% 64%
Tellers 6% 0% 1% 1% 0% 0% 0% 90%
Tellers 1% 0% 0% 0% 1% 0% 2% 94%
Services Overdrafts 2% 0% 1% 1% 19% 5% 8% 64%
Authorized
Personal2% 1% 0% 1% 11% 5% 11% 69%
Full Service 1% 0% 0% 0% 8% 1% 2% 88%
Entrance 13% 0% 3% 10% 58% 2% 0% 14% 0%
Legend: 0%-5% 5%-10% 10%-15% >15%
Dominant Paths - Business:
Unit Station 1 Station 2 Total
Parameter Tourism Teller Dominant Path
Service Time 12.7 4.8 17.5
Waiting Time 8.2 6.9 15.1
Total Time 20.9 11.7 32.6
Service Index 0.61 0.41 0.53
Dominant Paths - Private:
Unit Station 1 Station 2 Total
Parameter Banker Teller Dominant Path
Service Time 12.1 3.9 16.0
Waiting Time 6.5 5.7 12.2
Total Time 18.6 9.6 28.2
Service Index 0.65 0.40 0.56
Service Index = % time being served
17
Mapping the Offered Load (Bank Branch)
Business
Services
Private
Banking
Banking
Services
Department
Time Tourism Teller Teller Teller Comprehensive
8:30 – 9:00
9:00 – 9:30
9:30 – 10:00
10:00 – 10:30
10:30 – 11:00
11:00 – 11:30
11:30 – 12:00
12:00 – 12:30
Break
16:00 – 16:30
16:30 – 17:00
17:00 – 17:30
17:30 – 18:00
Legend:
Not Busy
Busy
Very Busy
Note: What can / should be done at 11:00 ?
Conclusion: Models are not always necessary but measurements are !
18
Conceptual Model: Burger King Bottlenecks
Bottleneck Analysis: Short – Run Approximations
Time – State Dependent Q-Net
3 Minimal:
Drive-thru
Counter
Kitchen
Add:
#4 Kitchen
#5 Help
Drive -thru
22
1
Service Engineering
Recitation 4, Part 1: Processing Networks.
An Emergency Department Example
The tutorial objective is to teach how to model a queueing network as a “Fork-Join network”.
Join Networks-ForkU
A fork-join network consists of a group of service stations, which serve arriving customers simultaneously and
sequentially according to pre-designed deterministic precedence constraints. More specially, one can think in terms of
"jobs" arriving to the system over time, with each job consisting of various tasks that are to be executed according to
some preceding constraints. The job is completed only after all its tasks have been completed. The distinguishing features
of this model class are the so-called "fork" and "join" constructs. A "fork" occurs whenever several tasks are being
processed simultaneously. In the network model, this is represented by a "splitting" of a task into multiple tasks, which
are then sent simultaneously to their respective servers. A "join" node, on the other hand, corresponds to a task that may
not be initiated until several prerequisite tasks have been completed. Components are joined only if they correspond to
the same job; thus a join is always preceded by a fork. If the last stage of an operation consists of multiple tasks, then
these tasks regroup (join) into a single task before departing the system.
ModelingU
We model our “fork-join network” using 4 specific flow-charts: activities, resources, activities plus resources, and
information. To draw these 4 flow charts one must list all resources of the network and all activities as well, and then
write which activity is using which resource. Next, one draws the flow charts, using the following “language”:
Chart Legend-FlowU
Often times, reality is too complex to capture with the above “language”. Then one must be creative, hence introduce,
ad hoc, the notation that will tell one’s specific story. (As an example, see page 2 where the “red-dot” is such a special
notation)
resource
decision
resources queue
synchronization queue
Job’s “flow”
“fork”
“join”
Labs
Treatment
Awaiting
evacuation
Administrative reception
Vital signs & Anamnesis
First
Examination
Imagine:
X-Ray, CT,
Ultrasound
Follow-up
Instruction prior
discharge
Waiting
hospitalized
Administrative discharge
Consultation
Decision
AA
AB
B
B
C
C
CC
C
Alternative Operation -
Ending point of alternative operation -
C
Figure 67: Activities flow chart in the ED
)
Administrative secretary
Nurse
Labs
Physician
Consultant
CT
X-Ray
Ultrasound
Alternative Operation -
Recourse Queue - Synchronization Queue –
Ending point of alternative operation -
C
A
A
A
A
AAB
B
B
B
B
C
C D
D
Figure 66: Resources flow chart in the ED
Alternative Operation -
Recourse Queue - Synchronization Queue –
Ending point of alternative operation -
C
PhysicianNurse Imaging LabOther
Follow-up
Instruction Prior
to Discharge Administrative
Release
Decision
Labs
Treatment
Administrative
Reception
Vital signs &
Anamnesis
Imaging:
X-Ray, CT,
Ultrasound
Treatment Consultation
First
Examination
AAAA
B
B
B
C
C
C
Figure 7: Activities-Resources flow chart in the EDEnding point of alternative operation -
Backgrounds
Receptions
Family doctor/ Internet/
Community
Clinical Information
Nursing Information
Nurse
Physician
Test tube and results
Labs
Shot result or
prognosis
Imagine
Clinical
Information
Consult
Collecting Result
Nurse
Clinical Information
Coordination with outsources
Nurse / ED Receptions
Figure 68: Information flow chart in the ED
8
Part 3: Applications and Results
The data is taken from an ED simulator written in Arena12.
Triage
External
X-Ray Multi-
Trauma
Trauma
Customer
Types
Departures
Released
Admitted
ArrivalsQueue
… and more
Private Prepaid
PrivatePrivate AT Private PreservationBusiness Business AT Business Preservation
Arabic Prepaid Arabic Prepaid BothersomeArabic PrivateRussian Technical Internet surfing DATA
Financial Overseas
Engineering Mobility Specials
Deliveries
1 24 5 810 11 12 1315161718 19 20 21 23 24 25 26 27 28 30
2104
2208 26264592 445 3411432049 1484 8734785 4658 411 444480 43 847 181 974
1253823 843343 1390109630 4454 1240792 5217 3793 49 735
3791 1433 193
58 125 685
1170
Private Prepaid
PrivatePrivate AT Private PreservationBusiness Business AT Business Preservation
Arabic Prepaid Arabic Prepaid BothersomeArabic PrivateRussian Internet surfing DATA
Overseas
1 24 5 810 11 12 1315161718 20 21 24 25
2104
2208 26264592 445 3411432049 1484 8734785 4658 411 444480 43 847 181 974
1253823 843343 1390109630 4454 1240792 3793 49 735
1433 193
Retail
PremierBusiness Platinum Consumer Loans Online BankingEBO TelesalesSubanco Case Quality Priority Service AST CCO Quick&Reilly BPS
1_1 1_57 1_15 2_3 2_63_1 3_5 5_1 6 8 10_1 11 12 13 15 17
10430 189932947 384 9840 157
54 1455 25755551 2372 597 91 195 342 3660 2173639 3251325 487 32 12 619 2352 3668 4844 447
VRU
Retail Premier Business PlatinumConsumer LoansOnline Banking EBOTelesales Subanco Summit
VRU
Retail Premier Business PlatinumConsumer Loans Online BankingEBOTelesales SummitOut
VRU
Retail Business
C
A
T
C
CC C C
C
CC
C
C
C
C
C
C
CC
C
C
C
C
C
C
AA
A
A
A
AAA A
A
A
A
A
A
A
T
OutOut Out Out OutOut Out OutOut Out
Out Out Out Out
55888
3611113 43320 160505194 7
36512 23633845 3792
845 103
120
314
3314
155
43
13
4
34
594 14017
547
191
16
1060 303174
1405
6
2373
432
242
3052 3908
7
23
12119
60
472131 20217
19
8
9
13 20 9
150
14 41611 56 6 106
8
7 58
30924
873
20863633 3536
562
867
1240
260
294
8
78
20
666
19835
3
1571
211148
29
213
3005
6
3671
18
11
19 356
126
10
46
4
12
3
5
1
2
1
node 1 node 2
node 3
C
A
CC
A
A
8742
1418611712
488
731
1804
1618 171426
10126
254
14688
125
9254
193
Deposit
(daily)
Check
Identification
Change
Agent
Fax
Query Trans
Saving
Accounts
Stock
Market
Poly
Credit
Error
43123
234
270
1352
35315
3900
7895
830
68
65
273
1063
231
768
133
1032
897
40
49
54
21
71
30
165
53
114
29
16
35
10
9
12
107
6
5
21
8
26091
4115
12317
962
1039
147
42
18
28
114
14
72
20
Analytical Models: Little’s Law, or
The First Law of Congestion
Input
(Customers,
units, …)
System Output
• λ = average arrival rate;
• L = average number within system;
• W = average time within system.
Little’s Law L = λW
Finite-Horizon Version
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!
!!!!!!
# customersA(T)=N
arrival
1
2
W7
0time
T
departure
Long-Run (Stochastic) Example
M/M/1: L =ρ
1− ρ=
λ
µ− λ, W =
1
µ− λ=1
µ
1
1− ρ.
23
Conceptual Model: The Justice Network, orThe Production of Justice
Queue
Mile Stone
Activity
Appeal
Proceedings
Closure
PrepareAllocateOpen File
Avg. sojourn time in months / years
Processing time in mins / hours / days
Phase Transition
Phase
24
Judges: Operational Performance - Base case
45 100
118
59
33
.
.
.
.
.
(6.2, 7.4) (13.5, 7.4)
(26.3, 4.5)
(12, 4.9)
(7.2, 4.6)
3
001
3
0
01
01
0
3
01
3
0
03
01
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30
Case Type 0 " Judge1
Case Type 01 !" Judge2
Case Type 3 #" Judge3
$" Judge4
!" Judge5
Aver
age
Num
ber
of
Month
s - W
Average Number of Cases / Month - !
25
3 Case-Types: Performance by 5 Judges
45 100
118
59
33
.
.
.
.
.
(6.2, 7.4) (13.5, 7.4)
(26.3, 4.5)
(12, 4.9)
(7.2, 4.6)
3
001
3
0
01
01
0
3
01
3
0
03
01
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30
Case Type 0 " Judge1
Case Type 01 !" Judge2
Case Type 3 #" Judge3
$" Judge4
!" Judge5
Aver
age
Num
ber
of
Month
s - W
Average Number of Cases / Month -
26
5 Judges: Performance by 3 Case-Types
(6.2, 7.4) (13.5, 7.4)
(26.3, 4.5)
(12, 4.9)
(7.2, 4.6)
.
.
.
.
.
3
001
3
0
01
01
0
3
01
3
0
03
01
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30
Case Type 0 " Judge1
Case Type 01 !" Judge2
Case Type 3 #" Judge3
$" Judge4
!" Judge5
Avg.
Month
s - W
Avg. Cases / Month -
27
Judges: Performance Analysis
3
001
3
0
01
01
0
3
01
3
0
03
01
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30
(6.2, 7.4) (13.5, 7.4)
(26.3, 4.5)
(12, 4.9)
(7.2, 4.6)
.
.
.
.
.
Case Type 0 " Judge1
Case Type 01 !" Judge2
Case Type 3 #" Judge3
$" Judge4
!" Judge5
Avg.
Month
s - W
Avg. Cases / Month -
28
Judges: Best/Worst Performance
(6.2, 7.4) (13.5, 7.4)
(26.3, 4.5)
(12, 4.9)
(7.2, 4.6)
3
001
3
0
01
01
0
3
01
3
0
03
01
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30
.
.
.
.
.
45 100
118
59
33
Case Type 0 " Judge1
Case Type 01 !" Judge2
Case Type 3 #" Judge3
$" Judge4
!" Judge5
Avg.
Month
s - W
Avg. Cases / Month -
29
Conceptual Fluid Model
Customers/units are modeled by fluid (continuous) flow.
Labor-day Queueing at Niagara Falls
• Appropriate when predictable variability prevalent;
• Useful first-order models/approximations, often suffice;
• Rigorously justifiable via Functional Strong Laws of Large
Numbers.
30
Empirical Fluid Model: Queue-Length at aCatastrophic/Heavy/Regular Day
Bank Queue
Catastrophic Heavy Load Regular
8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13Time of Day
0
10
20
30
40
50
60
Queue3
1
Empirical Models: Fluid, Flow
Derived directly from event-based (call-by-call) measurements. For
example, an isolated service-station:
• A(t) = cumulative # arrivals from time 0 to time t;
• D(t) = cumulative # departures from system during [0, t];
• L(t) = A(T )−D(t) = # customers in system at t.
Arrivals and Departures from a Bank Branch
Face-to-Face Service
0
50
100
150
200
250
300
350
400
8 9 10 11 12 13
time
cu
sto
mers
cumulative arrivals cumulative departures
number in
system
waiting
time
When is it possible to calculate waiting time in this way?
32
Mathematical Fluid Models
Differential Equations:
• λ(t) – arrival rate at time t ∈ [0, T ].
• c(t) – maximal potential processing rate.
• δ(t) – effective processing (departure) rate.
• Q(t) – total amount in the system.
Then Q(t) is a solution of
Q̇(t) = λ(t)− δ(t); Q(0) = q0, t ∈ [0, T ] .
In a Call Center Setting (no abandonment)
N(t) statistically-identical servers, each with service rate µ.
c(t) = µN(t): maximal potential processing rate.
δ(t) = µ ·min(N(t), Q(t)): processing rate.
Q̇(t) = λ(t)− µ ·min(N(t), Q(t)), Q(0) = q0, t ∈ [0, T ] .
How to actually solve? Mathematics (theory, numerical),
or simply: Start with t0 = 0, Q(t0) = q0.
Then, for tn = tn−1 +∆t:
Q(tn) = Q(tn−1) + λ(tn−1) ·∆t− µmin(N(tn−1), Q(tn−1)) ·∆t .
33
Time-Varying Queues withAbandonment and Retrials
Based on three paper with Massey, Reiman, Rider and Stolyar.
Call Center: a Multiserver Queue with
Abandonment and Retrials
Q1(t)
βt ψt ( Q1(t) − nt )+
βt (1−ψt) ( Q1(t) − nt )+
λt 2
Q2(t)
21 8. . .
nt
1
.
.
.
µt Q2(t)2
µt (Q1(t) nt)1
34
Primitives: Time-VaryingPredictability
λt exogenous arrival rate;
e.g., continuously changing, sudden peak.
µ1t service rate;
e.g., change in nature of work or fatigue.
nt number of servers;
e.g., in response to predictably varying workload.
Q1(t) number of customers in call center
(queue+service).
βt abandonment rate while waiting;
e.g., in response to IVR discouragement
at predictable overloading.
ψt probability of no retrial.
µ2t retrial rate;
if constant, 1/µ2 – average time to retry.
Q2(t) number of customers that will retry.
In our examples, we vary λt only, other primitives are constant.
35
Fluid Model
Replacing random processes by their rates yields
Q(0)(t) = (Q(0)1 (t), Q
(0)2 (t))
Solution to nonlinear differential balance equations
d
dtQ
(0)1 (t) = λt − µ1
t (Q(0)1 (t) ∧ nt)
+µ2t Q
(0)2 (t)− βt (Q
(0)1 (t)− nt)
+
d
dtQ
(0)2 (t) = β1(1− ψt)(Q
(0)1 (t)− nt)
+
− µ2t Q
(0)2 (t)
Justification: Functional Strong Law of Large Numbers,
with λt → ηλt, nt → ηnt.
As η ↑ ∞,
1
ηQη(t)→ Q(0)(t) , uniformly on compacts, a.s.
given convergence at t = 0
36
Sudden Rush Hour
n = 50 servers; µ = 1
λt = 110 for 9 ≤ t ≤ 11, λt = 10 otherwise
0 2 4 6 8 10 12 14 16 18 200
10
20
30
40
50
60
70
80
90Lambda(t) = 110 (on 9 <= t <= 11), 110 (otherwise). n = 50, mu1 = 1.0, mu2 = 0.1, beta = 2.0, P(retrial) = 0.25
time
Q1−odeQ2−odeQ1−simQ2−simvariance envelopes
37
Stochastic Framework: DS PERT/CPM
DS = Dynamic Stochastic (Fork-Join, Split-Match)
PERT = Program Evaluation and Review Technique
CPM = Critical Path Method
Operations Research in Project Management: Standard Successful.
New-York Arrest-to-Arraignment System
(Larson et al., 1993)
Lodged at
Precinct
(12 hrs.)
Arrive at
Precinct
(1 hr.)
Arrive at
Central
Booking
(5 hrs.)Off.
Arrives at
Complaint
Room
(6 hrs.)
Transmitted
to Albany
(10 hrs.)
Complaint
Sworn
(14 hrs.)
Rap Sheet
Received
(15 hrs.)
Arrives at
Courthouse
(39 hrs.)
Paperwork
Completed
(18 hrs.)
Arrestee
Arraigned
(48 hrs.)
Arrestee
Arrest
(0 hrs.)
fingerprints
CRM – task times are deterministic/averages (standard).
S-PERT (Stochastic PERT) – task times random variables.
DS-PERT/CPM – multi-project (dynamic) environment, with
tasks processed at dedicated service stations.
• Capacity analysis: Can we do it? (LP)
• Response-time analysis: How long will it take? (S-Nets)
• What if: Can we do better? (Sensitivity, Parametric)
• Optimality: What is the best one can do?
38
Stochastic Model of aBasic Service Station
Building blocks:
• Arrivals
• Service durations (times)
• Customers’ (im)patience.
• Customers’ returns (during service process, after service)
First study these building blocks one-by-one:
• Empirical analysis, which motivates
• Theoretical model(s).
Then integrate building blocks, via protocols, into (Basic) Models:
• Erlang-B/C (Arrivals, Services)
• Erlang-A (+ Abandonment), Erlang-R (+ Returns).
The models support, for example,
• Staffing Workforce, for which Basic Models are already useful;
and beyond:
• Routing Customers
• Scheduling Servers
• Matching Customers-Needs with Servers-Skills (SBR).
39
Interesting and Significant Additional Topics
• Stochastic Service Networks:
– Classical Markovian: Jackson and Gordon-Newell, Kelly/BCMP
Networks;
– Non-Parametric Network Approximations (QNA, SBR).
• Service Quality (Psychology, Marketing);
• Additional Significant Service Sectors: Healthcare, Hospital-
ity, Retail, Professional Services (Consulting), . . .; e-health,
e-retail, e-·, . . .;
• Convergence of Services and Manufacturing:
After-Sale or Field Support (life-time customer-value);
• Service Supply-Chains;
• New-Service Development (or Service-Engineering in Germany);
• Design and Management of the Customer-System Interface:
Multi-Media Channels; Appointments; Pricing; . . .
• Revenue Management (Finite Horizon, Call Centers, . . .)
93
Call Centers = Q’s w/ Impatient Customers15 Years History, or “A Modelling Gallery”
1. Kella, Meilijson: Practice ⇒ Abandonment important
2. Shimkin, Zohar: No data ⇒ Rational patience in Equilibrium
3. Carmon, Zakay: Cost of waiting ⇒ Psychological models
4. Garnett, Reiman; Zeltyn: Palm/Erlang-A to replace Erlang-
C/B as the standard Steady-state model
5. Massey, Reiman, Rider, Stolyar: Predictable variability ⇒
Fluid models, Diffusion refinements
6. Ritov; Sakov, Zeltyn: Finally Data ⇒ Empirical models
7. Brown, Gans, Haipeng, Zhao: Statistics ⇒ Queueing Science
8. Atar, Reiman, Shaikhet: Skills-based routing ⇒ Control mod-
els
9. Nakibly, Meilijson, Pollatchek: Prediction of waiting ⇒
Online Models and Real-Time Simulation
10. Garnett: Practice ⇒ 4CallCenters.com
11. Zeltyn: Queueing Science ⇒ Empirically-Based Theory
12. Borst, Reiman; Zeltyn: Dimensioning M/M/N+G
13. Momcilovic: Non-Parametric (G/GI/N+GI) QED Q’s
14. Jennings; Feldman, Massey, Whitt: Time-stable performance
(ISA)
94