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About this lesson
Process Capability consists of comparing the Voice of the Customer to the Voice of the Process. SPC Control Charts provide a quantitative measure of the Voice of the Process. This lesson explains the principle of process capability and the role of SPC in achieving and maintaining it.
Process Capability consists of comparing the Voice of the Customer to the Voice of the Process. SPC Control Charts provide a quantitative measure of the Voice of the Process giving us insight into the process capability.
When to use
Process capability is used to describe whether a process is consistently able to meet the customer expectations for performance. It is often used in industry as a surrogate for process quality. The SPC control chart information is often used to visualize process capability.
Process capability is the process metric that compares the voice of the customer to the voice of the process. The voice of the customer is expressed in terms of the process specification limits which are derived from the design of the process and the customer requirements. The voice of the process is derived from the process descriptive statistics. SPC control charts are linked to process capability. They are also derived from the process descriptive statistics and thy are used to maintain process control. Once a process capability is established, the SPC control charts become a primary management technique for maintaining that process capability.
A very important distinction to make is that the control limits on control charts are not the same as the spec limits for the process. They may occasionally be the same value, but they are still very different concepts and derived totally differently. The control limits are derived from the process and are based upon the process descriptive statistics; in particular, the mean and standard deviation. When the process performance changes, the control limits change. In contrast the spec limits are derived from the customer requirements and the design of the product or process to meet those requirements. It doesn’t matter what happens in the process, the spec limits are unchanged unless the customer requirements change. Ideally, the control limits are within the spec limits. In that case, as long as the process remains in control, the process should always deliver performance that meets the customer specification. However, a process can be in control but if the control limits are outside of the spec limits, the process will not be consistently meeting the customer requirements.
The method for calculating process capability varies depending upon whether the data is variable data or attribute data. Variable data is that which is measured on a scale. The value is always a number. With variable data, process capability is expressed using the indices Cp and Cpk for near term process performance and Pp and Ppk for long term process performance. The calculations for these will be discussed in other modules. The attribute data is category data such as pass/fail, yes/no, or a status indicator. This data is usually expressed in terms of a count or percentage of the data that is in the expected status and a count or percentage that is not. The process capability for attribute data will be derived from the yield values such as defects per unit, defects per opportunity, defects per million opportunity or parts per million. Again, these will be discussed in more detail in another module.
Process sigma is often used as a surrogate for process capability. Process sigma became popular with the widespread adoption of the six sigma quality improvement methodology. Process sigma also compares the customer requirements to the actual process performance. A six sigma process will have a process capability Cpk of 2, and a three sigma process will have a Cpk of 1. In many companies, the process capability or process sigma of a process is used as a measure of process quality.
Hints & tips
- Don’t fall into the trap of thinking that control limits are related to spec limits. The spec limits can be broader, narrower, or significantly displaced from the SPC control limits. That is why you need to look at process capability to determine if the process is really able to consistently meet the customer requirements. Process capability will combine the voice of the customer and the voice of the process.
- Attribute data process capability and variable data process capability are calculated differently. The literature loves to focus on the variable data capability indices – in part because these are much richer from a statistical perspective. However, in my experience a significant portion, possibly the majority, of data is attribute data. So you need to understand the process capability and control charts for that type of data also.
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