The Practical Difference Between Ppk and Cpk - c.ymcdn.com

1 The Practical Difference Between Ppk and CpkMark DiMartinoDirector Quality EngineeringAmgen Overview of Performance/Capability Indices What is Statistical Control? Types of Variation and the impact on Cpk/Ppk Examples23 Four Indices are Typically Used in Industry to Measure Process PerformancePotential Performance MeasureActual Performance MeasureAssumes Centered ProcessProcess Capability (Cp)Process Performance (Pp)Accounts for Process AverageProcess Capability Adjusted for Process Shift (Cpk)Process Performance Adjusted for Process Shift (Ppk)ASTM E22821 Standard Practice for Process and Measurement Capability Indices4 Each Index is a Measure of Process Performance Relative to a SpecificationThe output is an index standardized to a 3 SD range, meaning a Ppk/Cpk of indicates that there are 3 SDs Between the mean and the closest specification range.

2 Cp/Pp use the center of the specification ( , the target) in lieu of the mean. USLXLSLP rocess MeanProcess and Measurement VariabilitySpecification LimitsTarget3 SDsPpk = Does This Performance Measure Mean?2007 Estimated OOS (ppm) ppmThe performance measure can be converted to the probability of observing an out of specification (OOS) result. The higher the Index, the lower the probability of an OOS* *This interpretation applies to all process performance and process capability indices 6 The Index Value Is Driven by a Combination of the Mean, SD, and Specification LimitsUSLLSLT argetUSLLSLT argetUSLLSLT argetUSLLSLT arget0 bias, acceptable variabilitySlight bias, low variabilityBias, low variabilityLarge bias, very low variabilityCpk/Ppk = for each instance7 Per ASTM E2281: Process capability indices (Cp/Cpk) compare the variability of a process quality measure against product specifications or tolerances and assume the process is in a state of statistical control.

3 Process performance indices (Pp/Ppk) are useful in situations when the process in not in a state of statistical control. Mathematically: Cp/Cpk uses short term variation, for batch processes with a subgroup n=1 it is based on the average distance Between adjacent points. Pp/Ppk uses long term variation which is the normal Excel standard deviation is the Distinction Between Pp/Ppk and Cp/Cpk?What is Statistical Control? Statistical control or process stability is a state in which only common cause variation is evident. Common cause variation is the normal expected variation in a process that is: Due to the process itself Produced by interactions of variables within that process Present in all processes In statistical control (stable) provided it is the only variation present Predictable8 What Does it Mean to be Out of Statistical Control?

4 A process that is out of statistical control or unstable demonstrates evidence of special cause variation. Special Cause Variation Due to a specific, assignable cause Caused by special circumstances not inherent in the process. Generally easier to detect than common causes. Not predictable9 How Do I Determine if Special Cause Variation is Present? Statistical Process Control (SPC) techniques are used to determine the state of statistical control. The individuals control chart is the most typical SPC approach, with tests applied to identify special cause Rangeof Value36912151821242730 SampleAvg= ChartMoving Range ChartSingle Point outside 3 sigma limits6 in a row increasingor decreasing9 in a row above orbelow centerlineSingle point above the UCLfor the MR chartHow Does Statistical Control Relate to Cpk/Ppk?

5 Cp/Cpk provides an estimate of potentialprocess performance or capability How the process would perform in the absence of special cause variation. Pp/Ppk provides the actualprocess performance given all of the variance that is currently present in the process Pp/Ppk is generally <= Cp/Cpk11 But of Course it is Not That Simple There is a third type of variation that is often present, especially when considering batch processing Some statisticians call this Long term common cause variation Long term common cause variation is: Variation that a control chart may consider special cause, but is expected and generally tolerated in manufacturing Also called autocorrelation, where observations that are close together in time tend to look more similar than observations that are further apart in time.

6 Cp/Cpk generally underestimates the standard deviation in these situations, thereby overestimating of Long Term Common Cause Variability Campaign based manufacturing Same raw material lot used over multiple batches Equipment within a lab Analyst within a lab Column Packing Critical Reagents used over a long period of time Multiple purification lots from a single harvest lot13 Example 1: Common Cause Variation Example 1: Parameter demonstrates statistical control, all values exhibit random noise1460708090100110120130 Result3691215182124273033 SampleAvg= Range of Result3691215182124273033 SampleAvg= ChartMoving Range ChartLSLUSLT arget-3s+3sMean6080100120140 LSLUSLT arget-3s+ + + 2: Special Cause Evident15 Special cause variation leads to a discrepancy in Cpk/Ppk15 Individuals ChartMoving Range Range of Result36912151821242730333639 SampleAvg= +3sMean90001000011000 LSLUSLT arget-3s+3sMean90001000011000 Example 3: Long Term Common Cause16 Long term common cause variation leads to a discrepancy in Ppk/Cpk16 Individuals ChartMoving Range Range of Value012345678911131517192123252729 SampleAvg= Ppk and Cpk are both useful measures in assessing how a particular parameter is performing vs specification.

7 The concept of long term common cause variation needs to be taken into account when assessing statistical control. Understanding the nature of the underlying data is critical when interpreting Cpk or YOU!Any Questions?18