CHAPTER 4: FLOW TIME ANALYSIS
4.1 Objective
Chapter 3 introduced the three basic building blocks of process flow namely the (average) flow time,
(average) flow rate and (average) inventory. It is followed by a sequence of three chapters, 4, 5 and 6,
which examine each one of these measures individually. Chapter 4 is concerned with flow time analysis.
The major point we wish to drive here is that flow time can be fruitfully divided into two components
namely waiting time and activity time (theoretical flow time). The chapter analyses the theoretical time
in detail, postponing the analysis of waiting time to chapters 7 and 8.
4.2 Additional Suggested Readings
We assign Pizza Pazza (Kellogg case) as the major case for this module. (The Kristen Cookie Company,
(HBR case #9-686-093) is a fine alternative; see problem 2 at end of chapter). The case is simple and
accessible to most students, yet allows a rich discussion of the various factors that affect flow time such
as batch size, oven size and type, etc.
The discussion on theoretical flow time is based on the network flow framework of project management.
In class we often introduce the issue of parallel paths using the MBPF, Inc. example of building garages,
(see problem 3). The example is small enough that the notion of critical path can be conveyed without
going through the formal path computations.
4.3 Solutions to the Problem Set
Solutions to Chapter 4:
4.1
a.
The flow time of the process can be estimated by taking the average of the total time each
defendant spent in the system. This comes out to 120 minutes.
b.
To find the theoretical flow time we first find the average time for the activities “time
with judge” and “pay fine”. This can be done by averaging the ten observations given in the
table. These amount to 1.55minutes and 2.4 minutes respectively. Thus the theoretical flow
time is 1.55 + 2.4 = 3.95 minutes.
c.
The flow time efficiency is 3.95/120 = 3.3%
4.2
a.
The flow time of the two paths:
Path 1 (roof) : = 20+ 45 +45 +25+ 40 = 175 minutes
Path 2 (base): = 20+35+10 +30+25+ 40= 160 minutes
The critical path is path 1, and the flow time is 175 minutes.
b.
Activity 2 is not on the critical path
c.
The lead time will be reduced to 170 minutes.
4.3.
a.
b.
c.
d.
e.
The critical path involves the food preparation. The flow time is 4+18 + 12 = 34 minutes
Cart preparation is not on the critical path
The critical path will be reduced to 4+18 +10 = 32 minutes
The flow time is 34 + 4 = 38 minutes
Activity
A
F1
15
min
Process
And
Separate
10
min
Activity
B
F2
10
min
Activity
C
Loan
Officer
15
min
20
min
4.4
a.
The flow time is 50 minutes. The critical path is the one of F1 (process and separate,
Activity A, Activity B, Loan Officer).
b.
c.
Since activity A is on the critical path, the flow time will increase by 50%* 15 = 7.5
minutes, to 57.5 minutes.
4.5
a.
b.
The critical path is the one through the X-ray scanner.
The flow time is 30 + 40*1.5 + 60 = 150 seconds.
c.
d.
The flow time efficiency is 150/530 = 28%
The personal scanner is not on the critical path.