Continuous Process Improvement
Six Sigma
Six Sigma is an approach to quality that relies heavily on collection of data and identification of 'opportunities for defect'. It strives for a state of near perfection - 3.4 errors or defects in one million opportunities for error/defect. The level of defects is expressed as DPMO - Defects per Million Opportunities. A level of 3.4 DPMO equates to six standard deviations between the mean and the nearest specification limit. Standard Deviation is represented by a lower case Greek letter sigma - hence 'Six Sigma'.
In fact 3.4 DPMO equates to 4.5 sigma, but the Six Sigma approach argues that processes are capable of short term excellence to a factor of 1.5 sigma. Whilst customers would evaluate the process from a long term viewpoint, the role of the Six Sigma professional is to evaluate both long and short term capability and establish a strategy for improvement to best reach agreed performance measures.
A formula for identifying your sigma level in Excel would be:
= NORMSINV (1- ((No. of errors)/(No. of opportunities for error))) +1.5
The Six Sigma approach was invented by Motorola in 1986 and has since been adopted by General Electric and Allied Signal amongst others. It requires people with an in-depth knowledge of mathematics and statistics and can be expensive to implement. However large savings can be gained from such an approach, particularly in large global corporations.
When identifying opportunities for error, it is important to focus on the customer's expectation from the item in question. In Six Sigma terms these are the Critical to Quality factors - the key standards or specification that must be hit to ensure customer satisfaction. These should be measurable - Six Sigma deals in facts, in the objective rather than the subjective.
As an example: in terms of a data collection sheet, it would be possible to check each data item or field on the form for:
- spelling errors in the description
- whether data was always entered by the subject where necessary
- whether data was entered by the subject accurately or correctly
- whether data entry to a computerised information system was carried out where necessary
- whether data entry was accurate
- whether data was lost once entered on screen before it was saved to the database
- whether data was lost on saving from the data entry screen to the database
- whether data was corrupted subsequently
In terms of customer satisfaction there should also be a check that customers; or data subjects in this case, always understood the meaning of the data field.
That has given us 9 opportunities for error or defect for each field. If we apply Critical to Quality factors then it is possible to merge factors 4, 6 and 7 as from the customer's point of view data is either entered and saved or is not. But the breakdown may be important when analysing and eliminating the potential for error.
Six Sigma has two methodologies that can be used to drive towards lowering the DPMO:
DMAIC - used to improve an existing process, this stands for:
Define the scope of the current project, customer expectations and deliverables
Measure the current performance of the process. Calculate the mean, the minimum, the maximum and at least estimate the spread or variation if not calculate a standard deviation
Analyse the root causes of defects. Statistical tools and modelling are used to verify this. This stage should identify whether there is opportunity for improvement and, if so, identify some options
Improve the process. It may be necessary at this stage to trial different options before making a final decision - process owners and workers need to understand this
Control the continuation of the process. As we have noted elsewhere, processes tend to change incrementally over time. In this stage controls would track any changes in a log and monitor the process outputs, so that any detrimental effects would be made apparent and the cause would be known
DMADV is used where a new process is to be implemented or where DMAIC has been tried already without success. DMADV stands for:
Define the scope of the current project, customer expectations and deliverables
Measure customer requirements and the specifications the process must meet
Analyse the options to meet the above specifications
Design the process in detail
Verify that the design will perform and has the ability to meet customer needs
Whereas DMAIC is an incremental improvement to an existing process, DMADV is an approach aimed at getting things right in the first place.
Implementing Six Sigma
As already mentioned, Six Sigma requires an in-depth knowledge of mathematics and statistics. Six Sigma teams include 3 levels of staff - Green Belts, Black Belts and Master Black Belts.
Master Black Belts
Master Black Belts are the experts, managing a programme of Six Sigma projects each of which is managed by a Black Belt. They train and mentor Black Belts and Green Belts.
Black Belts
Black Belts are the Six Sigma team leaders, managing and coaching Green Belts. Typically they will have had at least 4 weeks of Six Sigma training, passing an exam to become a Black Belt. It is a full time post. An article by Thomas Pyzdek outlines 101 Things a Six Sigma Black Belt Should Know.
Green Belts
Green Belts have had some training in Six Sigma and have significant knowledge of the process that they are involved with improving. As such, Green Belts undertake their process improvement as a part of their full time jobs.
