Process Flow
from Prof. Goldsman’s lecture notes
Main Concerns…
Transform inputs into outputs
Internal measurement (process performance) versus
External measurement (customer satisfaction)
Productivity level (output/input)
Which one is controllable by the managers?
Remember …
• “Work smart, not hard”
• Systematic approach: find easier and better ways to
accomplish a task
• Steps: Identify problems -- Obtain facts -- Question the
facts -- Develop and test an improved method -- Simulate
the method -- Apply the method -- Review the method
– Let’s look into the flow…
Chart Types
Process Flow Chart
Left-Hand-Right Hand Charts (analyze the work
performed by one person at one specific WS)
Multiple Activity Chart (>= 2 objects/persons are
shown)
Work Distribution Chart (all activities/person/
department)
Operation Process Chart
Gantt Chart
Process Flow Diagram
Elementary
School
High
School
Middle
School
State
Exam
Fail
Pass
Georgia
Tech
Flow Diagram
Multiply Activity Chart
Gantt Chart
Process Flow Concepts
Process boundaries
Process flow rate: R (Throughput)
Process inventory: I (WIP, Inventory,
Bottleneck)
Unit flow time: T (on average, amount of time
a flow unit spends within the system)
Little’s Law
Flow unit
What is the flow unit in a kitchen in
restaurant, gas station pump, gas station, or
clothes store?
Process Flow Concepts
(Cont’d)
Inventory:
What are the inventory in a …
– University?
– Gas station?
– Kitchen in a restaurant?
• Flowtime for a particular item in a system:
= time it leaves system - time it enters system
Usually is on average, the amount of time a flow unit spends in
system
i.e. how long does a dollar remains in your checking account?
Process Flow Concepts
(Cont’d)
Throughput: rate of flow unit through a point
What is the throughput in …
A Kitchen in a restaurant?
Gas station?
Gas pump?
Clothes store?
Sneak Preview …
Let’s observe a bank teller,
What do we actually observe?
Sneak Preview … (Cont’d)
We can observe:
Events:
Service
Process
Arrival Process
Arrive at the “in” queue
Leave the “in” queue, starts service
Finishes service, enter the “out” queue
Leave the “out” queue
Sneak Preview … (Cont’d)
Averaged them out over time,
VOILA! Little’s Law!
•
•
•
•
Rate in = rate out (steady state)
Similar to physics (v x t = d)
Applicable to most systems even those with variability
Throughput rate x flowtime = inventory
R x T = I or I = R x T
Now, how can we determine the inventory in a
university?
Process Flow Concepts
(Cont’d)
Bottleneck:
Definition: reduce rate, reduce throughput
In a university?
Flowlines usually have bottlenecks -Assembly Line Balancing
Jobshops are more complex; less easily
applicable
Often become unclear when variability exists
Ok.. Little’s Law is awesome...
BUT, what about resource requirements?
How much capacity is “required”?
Estimating capacity requirement
Operation Capacity Required = R x
(resource/flow unit) -- utilization
Example: labor hours req’d =
200 pc/day x .25 hrs/pc = 50 hrs/day
or 7.14 workers -- at 100% utilization
Utilization isn’t 100%
Whenever the inventory goes to zero, the
process is idle, thus the utilization is less than
100%
Thus, estimate the utilization to be less than
100%
Revisit the previous example:
Suppose utilization is 75%, the labor hours
req’d is…
66.67 hrs/day or 9.52 workers
Look at a “network” of
processes… and apply
theoretical CPM
Process Flow
Process Flow Graph
Shows only the structure of the flow
Used to “map” the actual path and document
resource usage
Process Flow Chart
Shows the structure of the flows and detailed
operation information
Used to “document” times and distances, and
identify unquantified delays
How to improve flow time
efficiency?
Critical Path Concept
Calculate the longest path in the process
flow graph = Critical Path
Flow Time Efficiency
Theoretical Flow Time (T*)= Σ quantified process times
on the longest path
Actual Flow Time, T, is never less than T*, why?
Thus, the flow time efficiency is T*/T should be >, <, or =
1?
The inverse of flow time efficiency is called the “x
factor”
How to Improve It?
Focus on Critical Path!
Reduce work content of operations on critical
path
Increase lot size to reduce the number of setups
Reduce product rework
Reduce or eliminate inspection
“Balancing” the CP and non-CP
Dilbert Observation
If you are evaluated on the basis of flow
time efficiency, i.e., T*/T, one way to
make your efficiency better is to increase
T* while holding T constant
What do you think?
What about the maximum rate at
which units can be produced?
What is the Maximum R?
Now you are the Little’s Law
Expert!
Hopefully…