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1. Four Ways to Combine Six Sigma with
Theory of Constraints (TOC)
Many process improvement practitioners overlook the value
of combining Six Sigma with Theory of Constraints, says
contributor Abishek Soni. Here are four ways to harness the
synergy between Six Sigma and TOC.
Many companies adopt an either/or approach to process improvement and fail to
appreciate the benefits - and reap the rewards - of combining different process
improvement toolsets and approaches. I believe that Six Sigma and Theory of
Constraints, for instance, are important complementary approaches and not used often
enough together.
Why Six Sigma? Why TOC?
Six Sigma is a data driven improvement methodology which seeks to improve quality of
process outputs by removing causes of defects and reducing variation. It consists of
powerful statistical tools to measure, analyze and verify the performance of process.
Six Sigma projects follow DMAIC methodology to improve existing business processes.
DMAIC is composed of the following five phases:
DEFINE: This phase defines the problem statement and scope of the project.
MEASURE: During this phase key characteristics of process are collected.
ANALYZE: During this phase the root cause of the problem is investigated.
IMPROVE: During this phase improvement solutions are developed.
CONTROL: During this phase performance of the new process is monitored and
evaluated.
Theory of Constraints (TOC) is an ongoing improvement philosophy originally developed
by Eliyahu M. Goldratt that focuses on constraint management. ”A chain is no stronger
than its weakest link” is the fundamental tenet of theory of constraint.
TOC believes that the entire system is a collection of interrelated/interdependent
processes and each system has one or more constraint. A constraint is defined as any
process that limits the ability of entire system to achieve its intended goal.
TOC seeks to achieve POOGI (Process of ongoing improvement), by following Five
Focusing Steps mentioned below:
1. Identify the system constraint: Determine the weakest link or process of the
system which is limiting the ability of system to achieve its goal.
2. Decide how to exploit the constraint: Find ways to utilize the existing capacity
of constrained process to its fullest.
3. Subordinate all other processes to above decision: Align all the other process
to enable constraint to operate at maximum effectiveness.
4. Elevate the constraint: Make changes to break the constraint.
5. Go to step 1: If the constraint is broken go back to step 1 to identify new
constraint and repeat the subsequent processes.

2. Limitations of TOC and Six Sigma:
Six Sigma attempts to reduce variation in all the processes in order to achieve overall
improvement in system. System interdependencies are not taken into account and
processes are improved independently. As a result, there is plethora of potential projects
(improvement opportunities) in any system improvement initiative and it can be difficult to
identify which ones to tackle first.
Additionally, the propensity to reduce variation in all the processes sometimes make it
difficult to identify single or few processes of significant impact; ultimately escalating the
budget and time for the overall improvement program.
TOC provides good guideline to identify the constraint or bottleneck but its lack of
statistical tools to quantitatively measure and analyze the performance of the process
makes this methodology somewhat less effective.
Harness the Synergy Between Six Sigma and TOC:
After studying the above limitations it
seems both the improvement
methodologies are complementary to
each other. And if used in conjunction
significant amount of synergy can be
derived from them. TOC can be used to
identify the constraint (process of
significant impact) while statistical tools
of Six Sigma can be used to
quantitatively measure and analysis
process performance. In other words
TOC will enable an organization to
identify where to judiciously launch a Six
Sigma based improvement project.
Here are four areas where TOC and Six
Sigma could be used in conjunction:
Opportunity #1:Identify the significant opportunity
Start with first step of TOC. Identify the constraint to discover the bottleneck process in the
overall system. Once the process of significant impact is identified introduce Six Sigma.
DEFINE phase of Six Sigma methodology will concentrate only on the identified constraint or
bottleneck process (restricting the overall improvement scope, budget and time) and
correspondingly determine the most significant CTQ. Use MEASURE phase tools to
determine performance standard, collect process data and conduct measurement system
analysis.
The most significant improvement opportunity with well-defined scope and performance
standard is the final deliverable of this phase.

3. Opportunity #2: Analyze the Problem
Use ANALYZE phase tools to establish capability of bottleneck process. Leverage tools
such as ANOVA, Regression analysis, Pareto, Ishikawa diagram to identify the sources
of variations and root cause of the problem. Complement analysis with TOC step,
identifying ways to exploit the constraint or bottleneck process to its full capacity. This
will ensure that existing capacity is utilized without any additional investment.
Opportunity #3: Develop the Solution
Generate possible solutions for the problem using various techniques of IMPROVE
phase such as DOE, Poka Yoke, Pugh Matrix, FMEA etc… Bolster the solution with TOC
step; subordinate all the other processes to exploited constraint. This will ensure that all
other processes of the system are aligned with solution identified. If sufficient investment
is available proceed with TOC step to elevate the constraint i.e. remove the constraint of
the system by increasing its capacity.
Opportunity #4: Verify & Continuously Improve
Gather feedback on the performance of the new process using CONTROL phase tool
control charts. Continue the pursuit of excellence and begin with first step of TOC to
discover new constraint.
References:
 The Goal by
Eliyahu M.
Goldratt
 The Six Sigma
Way by Peter
S. Pande,
Robert P.
Neuman,
Roland R.
Cavanagh

4. About Abhishek Soni
Abhishek Soni is a certified six sigma black belt and an MBA graduate
from XLRI, Jamshedpur, India. He has around six years of experience in
process improvement function in financial services domain in IT industry.
He is currently working as a process improvement consultant in a
reputed Indian IT firm. He can be reached at mail2_abhishek@sify.com.
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