Noted Quality expert Joseph Juran has criticized Six Sigma as ‘a basic version of quality improvement’.
It has been suggested that Six Sigma is effective at what it is intended to do, but that it is ‘narrowly designed to fix an existing process’
and does not help in ‘coming up with new products or disruptive technologies.’
When Six Sigma is used as a cost cutting program, it has been shown to stifle new product innovation.
While 3.4 defects per million might work well for certain products/processes, it might not be ideal for others.
A pacemaker might need higher standards, for example, whereas a direct mail advertising campaign might need less.
The basis and justification for choosing 6 as the number of standard deviations is not clearly explained.
Six Sigma uses a variety of tools, some of which are referred to below.The 5 Whys
The five whys is a question asking method used to explore the cause and effect relationships underlying a particular problem.
Ultimately, the goal of applying the 5 Whys method is to determine a root cause of a defect or problem.
The following example demonstrates the basic process.
My car will not start. (the problem)
Why? The battery is dead. (first why)
Why? The alternator is not functioning. (second why)
Why? The alternator has broken beyond repair. (third why)
Why? The alternator is well beyond its useful service life and has never been replaced. (fourth why)
Why? I have not been maintaining my car according to the recommended service schedule. (fifth why, root cause)
The five iterations are not gospel; rather, it is postulated that five iterations of asking why is generally sufficient to get to a root cause.
The real key is to encourage the trouble shooter to avoid assumptions and logic traps and instead to trace the chain of causality in direct increments from the effect through any layers of abstraction to the first or root cause.
The technique was originally developed by Sakichi Toyoda and was later used within Toyota Motor Corporation during the evolution
of their manufacturing methodologies.
It is a critical component of problem solving training delivered as part of the induction into the Toyota Production System.
The architect of the Toyota Production System, Taiichi Ohno, described the 5 whys method as ‘... the basis of Toyota's scientific approach... by repeating ‘why’ five times, the nature of the problem as well as its solution becomes clear.’
The tool has seen widespread use beyond Toyota, and is now also used within Six Sigma.
Some consider the 5 Whys to be a powerful mnemonic for engineers or technically savvy individuals to help get to the true causes of problems.
It has also been criticized as being too basic a tool to analyze root causes to the depth that is needed to ensure that the causes are fixed.
Reasons for this criticism include:
These can be significant problems when the method is applied through deduction only.
On-the-spot verification of the answer to the current ‘why’ question, before proceeding to the next, is recommended as a good practice to avoid these issues.
A CTQ Tree (Critical to Quality Tree) is used to decompose broad customer requirements into more easily quantified requirements.
CTQ Trees are often used in the Six Sigma methodology.
A CTQ tree decomposes a general, hard to measure requirement on the left, through several steps, into more specific, easy to
measure variables on the right.
It can be a simple, useful tool when the customer requirements are unspecified, broad, or difficult to measure.
Sometimes this decomposition can be via mathematically combinable numbers (profit decomposed into ‘price’ and ‘cost’),
but sometimes the variables cannot be directly combined mathematically (customer service decomposed into ‘short wait’ and ‘friendly staff’).
A CTQ tree is a quick tool generally used near the beginning of a project, with a focus of discovering measurable critical-to-quality requirements, and not fully exploring the causes that influence those measures.
It should not be confused with the cause-and-effect diagram, sometimes referred to as the fishbone or Ishikawa diagram.
It is a risk assessment technique for systematically identifying potential failures in a system or a process.
It is widely used in the manufacturing industries in various phases of the product life cycle.
This means the ways, or modes, in which something might fail.
Failures are any errors or defects, especially ones that affect the customer, and can be potential or actual.
This refers to studying the consequences of those failures.
In FMEA, failures are prioritized according to how serious their consequences are, how frequently they occur and how easily they can be detected.
An FMEA also documents current knowledge and actions about the risks of failures, for use in continuous improvement.
FMEA is used during the design stage with an aim to avoid future failures.
Later it’s used for process control, before and during ongoing operation of the process. Ideally, FMEA begins during the earliest conceptual stages of design and continues throughout the life of the product or service.
The purpose of the FMEA is to take actions to eliminate or reduce failures, starting with the highest-priority ones.
It may be used to evaluate risk management priorities for mitigating known threat-vulnerabilities.
FMEA helps select remedial actions that reduce cumulative impacts of life-cycle consequences (risks) from a systems failure (fault).
The basic process is to take a description of the parts of a system, and list the consequences if each part fails.
In most formal systems, the consequences are then evaluated by three criteria and associated risk indices:
An FMEA simple scheme would be to have three indices ranging from 1 (lowest risk) to 10 (highest risk).
The overall risk of each failure would then be called the ‘Risk Priority Number (RPN) and the product of Severity (S), Occurrence (O), and Detection (D) rankings: RPN = S × O × D.
Hence, the RPN will range from 1 to 1000.
It is used to prioritize all potential failures to decide upon actions leading to reduce the risk, usually by reducing likelihood of occurrence and improving controls for detecting the failure.
There are generally two classes of software used to support Six Sigma:
Analysis tools, which are used to perform statistical or process analysis
Program management tools, used to manage and track a corporation's entire Six Sigma program
There are too many to review here.
More information can be found in the online encyclopaedia Wikipedia.
Under PRINCE2® 2009 [see ‘The Complete Project Management plus PRINCE2’] planning is covered by the Plans theme.
The purpose of the Plans theme is to facilitate communication and control by defining the means of delivering the products (the where and how, by whom, and estimating the when and how much).
[see Plans - Purpose]
PRINCE2® is a Registered Trade Mark of the Office of Government Commerce in the United Kingdom and other countries.