CHAPTER ELEVEN
BUSINESS SIMULATION
Dr. Rami Gharaibeh
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benefits
simulation is useful for training, persuasion, and
analysis.
Simulation is also useful for model validation—for finding
and fixing errors in a model.
Dr. Rami Gharaibeh
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simulating a business process
model
Example
you seek to enhance customer satisfaction
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simulating a business process
model
Traditionally Mykonos management has thought about
customer satisfaction in terms of the quality of the
dishes prepared
You intend to use your new responsibility to consider
customer satisfaction more broadly.
want to look at customer wait times and the impact of
waits
on satisfaction.
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simulating a business process
model
Customers have many waitings
They wait for a table to be available. They wait for water,
and bread. They wait for the staff to take their orders.
They wait for their food, and they wait for their bills.
You are concerned that all this waiting makes customers
dissatisfied
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simulating a business process
model
You will create two models
the customer dining process. how customers arrive, are
seated, have their orders taken, and so on
customer satisfaction. how food quality, wait times, and
other factors contribute to customer satisfaction and
how word of mouth and restaurant reviews affect the
view of potential customers and, ultimately, a
restaurant’s success.
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simulating a business process
model
Working with the models, you could examine the effect of
different policies on wait times. And the two models
could help communicate the results to the restaurant
general manager, ultimately persuading him to
change his policies and reduce the wait times.
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simulating a business process
model
Working with the models, you could examine the effect of
different policies on wait times. And the two models
could help communicate the results to the restaurant
general manager, ultimately persuading him to
change his policies and reduce the wait times.
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simulating a business process
model
After several sessions with the SME the models were
developed
The simulation shows that each customer party waits 46
minutes on average. This 46 minutes includes all the
waits
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simulating a business process
model
The following figure shows an initial simulation result, a
breakdown of the average times spent by customers,
organized by activity
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simulating a business process
model
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simulating a business process
model
Wait times vary over the course of the week. Mondays
and Tuesdays see far fewer customers than Fridays
and Saturdays, and far shorter waits.
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simulating a business process
model
Dr. Rami Gharaibeh
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simulating a business process
model
Wait times also vary over the course of a single evening.
The wait times are shortest both early and late in the
evening and longest in the middle of the evening,
when the restaurant is busiest.
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simulating a business process
model
Dr. Rami Gharaibeh
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simulating a business process
model
after validating the model, we will start the analysis
What happens if the staffing is doubled?
What happens on Friday nights if there are two hosts
available to greet and seat customers instead of one,
twice as many servers waiting tables, twice as many
chefs cooking meals, and twice as many bartenders
mixing drinks?
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simulating a business process
model
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simulating a business process
model
Why is the reduction in wait times so modest even when
the staff is doubled?
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simulating a business process
model
The law of diminishing returns
If we are constrained in one resource, the increase in
another resource will bring diminishing returns
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simulating a business process
model
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simulating a business process
model
What happens if the staffing is doubled, and the number
of tables is doubled and the kitchen capacity is
doubled?
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simulating a business process
model
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simulating a business process
model
Most of the waits have disappeared.
the obvious conclusion from this experiment:
The restaurant is just not big enough for the demand on
busy nights.
But, doubling the capacity is not realistic !!!
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simulating a business process
model
a more realistic alternative is to change our policy
What if we ask customers to make reservations on busy
nights?
Here are the results
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simulating a business process
model
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simulating a business process
model
People are in fact waiting less.
But that reduced wait comes at a cost: the restaurant is
serving fewer people and the bar is serving far fewer
drinks to people who are waiting.
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simulating a business process
model
Many other experiments are possible.
What if the size of the waiting list is no more than three
parties at a time?
What if we staff a single additional server on Fridays and
Saturdays?
What if we cross-train the servers, so they could perform
as hosts if the host was busy seating people?
What if we only seated smaller parties—those of six
people or fewer?
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CHAPTER ELEVEN
activities, resources, and jobs
Preparing a model for simulation requires some
additional work beyond what is required to create a
static, non-simulated model.
But more significantly than the extra work, preparing a
process model to be simulated requires additional
knowledge.
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activities, resources, and jobs
To create a business process simulation you must
understand activities, resources, and jobs and the way
these three interact with each other.
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CHAPTER ELEVEN
activities, resources, and jobs
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CHAPTER ELEVEN
activities, resources, and jobs
activity is a single step in a larger business process.
A resource is a person who performs the activity
Server is a role the person plays.
A job is something that flows through the process, being
worked on by resources and flowing from activity to
activity
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Job cycle
A job is created at a start event and flows over sequence
flows and message flows from activity to activity until
an end event is reached.
When a job reaches an activity, one of two things can
happen. Either a resource is available to work the job
or no resource is available and the job must wait until a
resource is available
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Job cycle
While the resource is performing an activity he is not
available to do anything else.
When the resource is finished the activity, the job and the
resource separate.
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Job cycle
The resource is available to do something else: the same
activity for another job, another activity for the same
job, or another activity in another process.
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Job cycle
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Collecting statistics
The simulation engine collects individual statistics as the
simulation progresses—statistics about activities,
resources, and jobs.
These statistics are aggregated into the simulation
results
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Collecting statistics
The engine collects statistics on each activity, each
resource, and each job.
Activity statistics include how many times each activity
was performed, the average duration of each activity,
the total cost of each activity.
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Resource statistics
Resource statistics include the utilization of each
resource, the total amount of work performed by
each resource,
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Job statistics
Job statistics include the total cycle time of each job, the
total touch time of each job,
 Job cycle time; for example, some jobs take two weeks to
process in the endto-end simulation, but in the real world
everything is done within four days
 Job touch time; for example, some jobs are only worked by
resources for 20 minutes, but in the real world every job is
worked for at least an hour
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Collecting statistics
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Collecting statistics
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CHAPTER ELEVEN
activity durations page 303
To be simulated, activities need additional attributes.
Duration is one such attribute. When a job arrives at
an activity, how long does the resource work on it?
Each activity has a duration attribute that indicates
how long jobs need to be worked.
 duration is a constant value, such as 10 minutes for every
job that is worked by this activity. But duration can also
vary, and it can vary in three different ways.
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CHAPTER ELEVEN
activity durations
Activity duration can vary in three different ways.
First, duration can vary depending on the details of the
job
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activity durations
The job that models the party of eight needs to be
different from the job that models a party of two.
For this simulation, jobs need their own model-custom
attribute, party Size.
A job representing a party of eight will have a party size
value of 8, and a job representing a party of two will
have a value of 2.
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CHAPTER ELEVEN
activity durations
Second, the duration of the activity can vary depending
on the details of the resource.
A skilled server will serve drinks quicker than a novice
because she remembers who ordered which drink.
The simulation needs to know that this server is quick,
that one is average, and this other one is slow.
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CHAPTER ELEVEN
activity durations
For this model, resources need a model-custom attribute,
skill Level, to keep track of skill.
Values of skill level can then be 1 for an average skill, 0.8
for a quick server, and 1.2 for a slow one.
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CHAPTER ELEVEN
activity durations
Suppose we want the duration of Take Dinner Order to
consider both the skill of the server and the size of the
party. How do we combine those elements?
We need to encode the duration as a formula, perhaps
like the duration in the following slide
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CHAPTER ELEVEN
activity durations
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CHAPTER ELEVEN
activity durations
Third, the duration of the activity can vary randomly.
Consider the activity Cook Dinner. The duration of
Cook Dinner might vary from 20 minutes to 45
minutes, depending on what is ordered.
When durations vary randomly, modelers often employ a
uniform distribution. Each duration in a uniform
distribution within the range is equally likely; 20
minutes is just as likely as 33 minutes and just as likely
as 40 or 45.
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CHAPTER ELEVEN
activity durations
https://samehar.wordpress.com/2010/03/05/distributions/
comment-page-1/
When durations vary randomly, modelers often employ a
uniform distribution. Each duration in a uniform
distribution within the range is equally likely; 20
minutes is just as likely as 33 minutes and just as likely
as 40 or 45.
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CHAPTER ELEVEN
activity durations
It is also possible to use other distributions: a triangular
distribution that peaks somewhere in the middle,.
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activity durations
a standard (bell curve) distribution
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activity durations
or something more sophisticated.
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activity durations
The statistical distributions (e.g., normal, log-normal,
exponential, etc.) often lead to higher model fidelity,
but they are usually difficult for SMEs to understand.
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CHAPTER ELEVEN
work time and delay time
The duration of an activity specifies the work time—the
time a person is actually working on a job.
But a job can also experience a delay while it is at an
activity, time when no one is actually working on the
job. A resource delay occurs when there is no
resource available to work a job.
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CHAPTER ELEVEN
work time and delay time
What happens if the resource starts working on the job
then the resource becomes unavailable?
We use the resourceShift attribute
resource- Shift indicates that the job should be given to
another resource if the original one becomes
unavailable.
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CHAPTER ELEVEN
work time and delay time
In some situations, the resource should be the same as
the one used at the prior activity.
The job waits until that particular resource is available.
The activity attribute consistentResource is used to
indicate this situation
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CHAPTER ELEVEN
work time and delay time
In addition to resource delays, a job can experience
an intrinsic delay.
An intrinsic delay is a delay that occurs as part of the
normal work of the job.
It might take 40 minutes to prepare the dinner: 25
minutes of work and 15 minutes of intrinsic delay.
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CHAPTER ELEVEN
work time and delay time
When working a single activity, a job can experience both
a resource delay and an intrinsic delay.
Cook Dinner includes an intrinsic delay as part of the
nature of preparing food. If many dinner orders arrive
at once, the chef might have more work than he can
do, and some of the dinners suffer delays—resource
delays—until the others are finished. (Or the chef
could be limited by the physical resources of the
kitchen)
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CHAPTER ELEVEN
work time and delay time
Intrinsic delays and resource delays are both delays. In
both situations, the job is waiting and no work is being
performed.
But they are modeled differently.
If an activity has an intrinsic delay, its intrinsicDelay
attribute will indicate the amount of time that the job is
delayed. Resource delay is not specified in an activity
attribute. Instead it is a job waiting for a resource to
work it.
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CHAPTER ELEVEN
work time and delay time
A delay can also occur in a flow, either in a sequence
flow between activities in the same pool or in a
message flow between pools.
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CHAPTER ELEVEN
simulating exclusive gateways
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simulating exclusive gateways
In the midst of a simulation, when a job arrives at an
exclusive gateway, how does the simulation engine
decide which way to send the job? Does this job
represent a party that is ordering drinks or one that will
move straight to ordering dinner
There are three alternative modeling approaches to this
question.
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simulating exclusive gateways
The simplest approach is for each of the outgoing
sequence flows from the gateway to indicate a
percentage of jobs.
Each sequence flow has a conditionExpression attribute
indicating whether the sequence flow will be taken.
One of the sequence flows is a default; it is the flow
taken if none of the others is chosen.
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simulating exclusive gateways
For example, suppose 60 percent of the parties order drinks before
dinner and 40 percent do not.
A job that arrives at the gateway will have a 60 percent chance of
taking the lower path and a 40 percent chance of taking the upper
path. Each job is evaluated differently, so in the midst of a
simulation run, it is possible for four jobs in succession to beat the
odds and all take the upper path. But over the course of hundreds
of jobs, the actual results will be close to 60/40.
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CHAPTER ELEVEN
simulating exclusive gateways
A second way of modeling how an exclusive gateway determines the
outgoing path is to examine an attribute of the job.
Suppose that each job in this model had a beforeDinnerDrink
attribute, indicating whether the party will order drinks before
dinner. For some of the jobs this attribute is true and for others it
is false. Then for each job, the outgoing sequence flows from
Order Drinks? will examine the value of this attribute. Those with
a value of false will be sent along the upper path, and those with a
value of true will be sent on the lower path.
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simulating exclusive gateways
There are two advantages of driving a gateway using a
job attribute instead of probabilities evaluated on the
sequence flow.
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simulating exclusive gateways
1
Job statistics can be analyzed and sorted by the job
attribute.
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simulating exclusive gateways
2
Attribute-based scenarios can be created.
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simulating exclusive gateways
There is also a third way to model an exclusive gateway:
combining the two approaches and making the
gateway split depend on both an attribute of the job
and on a random percentage.
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simulating exclusive gateways
the conditionExpression attribute of the lower outgoing
sequence flow from the Order Drinks?
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