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15/12/2017 09:34:23 AM

Six-Sigma Quality

Six Sigma refers to the philosophy and methods companies such as General Electric and Motorola use to eliminate defects in their products and processes. A defect is simply any component that does not fall within the customer’s specification limits. Each step or activity in a company represents an opportunity for defects to occur and Six-Sigma programs seek to reduce the variation in the processes that lead to these defects. Indeed, Six-Sigma advocates see variation as the enemy of quality, and much of the theory underlying Six Sigma is devoted to dealing with this problem.

A process that is in Six-Sigma control will produce no more than two defects out of every billion units. Often, this is stated as four defects per million units, which is true if the process is only running somewhere within one sigma of the target specification.

One of the benefits of Six-Sigma thinking is that it allows managers to readily describe the performance of a process in terms of its variability and to compare different processes

using a common metric. This metric is defects per million opportunities (DPMO). This DPMO calculation requires three pieces of data:

    1.   Unit. The item produced or being serviced.

    2.  Defect. Any item or event that does not meet the customer’s requirements.

    3.   Opportunity. A chance for a defect to occur.

A straightforward calculation is made using the following formula:

Example:

The customers of a mortgage bank expect to have their mortgage applications processed within 10 days of filing. This would be called a critical customer requirement, or CCR, in Six-Sigma terms. Suppose all defects are counted (loans in a monthly sample taking more than 10 days to process), and it is determined that there are 150 loans in the 1,000 applications processed last month that don’t meet this customer requirement. Thus, the DPMO = 150/1000 × 1,000,000, or 150,000 loans out of every million processed that fail to meet a CCR. Put differently, it means that only 850,000 loans out of a million are approved within time expectations. Statistically, 15 percent of the loans are defective and 85 percent are correct. This is a case where all the loans processed in less than 10 days meet our criteria. Often there are upper and lower customer requirements rather than just a single upper requirement as we have here

Six – Sigma Methodology

The standard approach to Six – Sigma projects is the DMAIC methodology developed by General Electric, described below:

1.   Define (D)

-        Identify customers and their priorities.

-        Identify a project suitable for Six-Sigma efforts based on business objectives as well as customer needs and feedback.

-        Identify CTQs (critical-to-quality characteristics) that the customer considers to have the most impact on quality.

2.   Measure (M)

-        Determine how to measure the process and how it is performing.

-        Identify the key internal processes that influence CTQs and measure the defects currently generated relative to those processes.

3.   Analyze (A)

-        Determine the most likely causes of defects.

-        Understand why defects are generated by identifying the key variables that are most likely to create process variation.

4.   Improve (I)

-        Identify means to remove the causes of defects.

-        Confirm the key variables and quantify their effects on the CTQs.

-        Identify the maximum acceptance ranges of the key variables and a system for measuring deviations of the variables.

-        Modify the process to stay within an acceptable range.

5.   Control (C)

-        Determine how to maintain the improvements.

-        Put tools in place to ensure that the key variables remain within the maximum acceptance ranges under the modified process.