Posts in category Six Sigma


Six SigmaTerms

Capability Ratio

The capability ratio is the inverse of the Cp index. Remember that the Cp index is the specification spread divided by the process spread of six standard deviations. Flip that formula around and you would be dividing the process spread by the specification spread. In the case of a capability ratio, you would be looking for a smaller-is-better characteristic. With the Cp index, you are looking for a larger-is-better characteristic.

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Six SigmaTerms

Capability Study

As mentioned in the course, there are two ways to make a bad part, or to make for an unhappy customer experience when dealing with continuous data. Either the process variation centering is off, or the dispersion (fatness, or slop in the process) is too wide. A capability study will tell you with two numbers represented by either Cp Cpk, or Pp Ppk whether your process is capable of meeting customer requirements in either the short term or the longer-term.

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Six SigmaTerms

Cause-Effect Diagram

A cause/effect diagram was first known by the name fishbone diagram because it looks like the skeleton of a fish. It was first made popular by Dr. Ishikawa back in the late 1970s and early 80s. Usually the cause/effect diagram is drawn on a large whiteboard or a large flipchart. The effect is usually written at the 3 o’clock position. A horizontal line divides the whiteboard into two equal parts.

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Six SigmaTerms

Charter

A project charter is a document that contains the basic elements (i.e., business case, problem statement, and scope) of the improvement project and answers the following questions:

1. The business case describes what the project does, how it impacts the strategic business objectives, is used as a motivational tool that describes why the project is worth doing, and it explains the consequences of not doing the project.

2. The problem statement is specific and measurable (quantifiable). It is an indication of how long the problem has existed, describes the impact to the organization, and describes the gap between the current state and the desired state.

3. The scope defines what the team is going to focus their process improvement efforts on, and it will identify those things that are out of scope so that the team doesn’t creep toward areas that are beyond the initial intent of the project.

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Six SigmaTerms

Chi-square Distribution

A chi-square test is a statistical tool used to test for independence or dependence (or goodness-of-fit) between random variables taken from different populations. A chi-square test could also be used for testing for goodness between an observed frequency distribution and an expected frequency distribution. With the chi-square test you are comparing a target variance with an observed variance. It is used to test the independence of two nominal variables. Remember that nominal variables are names or categories only.

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