Page 207 - The Six Sigma Project Planner

throughput, then it can be considered to be a constraint. For example, if an upstream
process average yield is enough to feed the constraint on the average, it may still present
a problem. An upstream process producing 20 units per day with an average yield of
90% will produce, on average, 18 good units. If the constraint requires 18 units, things
will be OK about 50% of the time, but the other 50% of the time things won’t be OK.
One solution to this problem is to place a work-in-process inventory between the
process and the constraint as a safety buffer. Then on those days when the process yield
is below 18 units, the inventory can be used to keep the constraint running. However,
there is a cost associated with carrying WIP inventory. A Six Sigma project that can
improve the yield will reduce or eliminate the need for the inventory and should be
considered even if it doesn’t impact the constraint directly, assuming the benefit-cost
analysis justifies the project. On the other hand, if an upstream process can easily make
up any deficit before the constraint needs it, then a project for the process will have a
low priority.
Knowing the project’s throughput priority will help you make better project selection
decisions by helping you select from among project candidates. Of course, the
throughput priority is just one input into the project selection process; other factors may
lead to a different decision. For example, impact on other projects, a regulatory
requirement, a better payoff in the long term, etc.

Multitasking and Project Scheduling
A Six Sigma enterprise will always have more projects to pursue than resources to do
them. The fact that resources (usually Black Belts or Green Belts) are scarce means that
projects must be scheduled, i.e., some projects must be undertaken earlier than others.
In such situations, it is tempting to use multitasking of the scarce resource. Multitasking
is defined as the assignment of a resource to several priorities during the same period of
time. The logic is that by working on several projects or assignments simultaneously,
the entire portfolio of work will be done more quickly. However, while this is true for
independent resources working independent projects or subprojects in parallel, it is not
true for a single resource assigned to multiple projects or interdependent tasks within a
project.

Consider the following situation. You have three Six Sigma projects: A, B, and C. A
single-tasking solution is to first do A, then B, and then C. Here’s the single-activity
project schedule.

A B C
Complete in Week 10 Complete in Week 20 Cmplete in Week 30

If each project takes 10 weeks to complete, then A will be completed in 10 weeks, B in 20
weeks, and C in 30 weeks. The average time to complete the three projects is calculated
as follows:

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