ST: 4.6.3. Interpret Cp and Cpk
Revision Date: 2005-09-07 The Cp and Cpk indices are the primary capability indices. Cp shows whether the distribution can potentially fit inside the specification, while Cpk shows whether the overall average is centrally located. If the overall average is in the center of the specification, th ...
- ST: 4.6. Calculate and interpret the capability indices
Revision Date: 2005-09-07 This step describes the key capability indices. 4.6.1. Calculate Cp. 4.6.2. Calculate Cpk 4.6.3. Interpret Cp and Cpk. 4.6.4 Calculate Cpu and Cpl Practical Tools for Continuous Improvement discusses other indices, such as Cpm and Cr. The above article ...
- ST: 4.6.4. Calculate Cpu and Cpl
Revision Date: 2005-09-07 Cpu and Cpl are the Cpk values calculated for both Z values. Therefore, Cpu is: For the example: Cpl is: For the example: From Cpu and Cpl, it is evident that the smaller value for the example is Cpu, which is the same value as Cpk. By comparing Cpu to Cpl, it is ...
- ST: 3 When is it used
Revision Date: 2005-09-06 Use the standard method for calculating capability analysis when you can answer "yes" to all of the following questions: 1. Is it necessary to understand how the system performs in comparison to specification limits? Specifications or requirements must be availabl ...
- ST: 4.2. Calculate the estimated standard deviation
Revision Date: 2005-09-06 The next stage is to calculate the position of the tails of the distribution that has just been drawn. However, in order to calculate the position of the tails, the standard deviation is required. In this version of capability analysis where data has been collected ...
- ST: 1 What is it
Revision Date: 2005-09-06 Capability analysis is a set of calculations used to assess whether a system is statistically able to meet a set of specifications or requirements. To complete the calculations, a set of data is required, usually generated by a control chart; however, data can be c ...
- ST: 4 How is it made
Revision Date: 2005-09-06 These steps assume that variables data has been collected over time, and that a control chart and histogram have been completed. The control chart should show no special causes, and the histogram should reveal that the data is normally distributed. In the example, ...
- ST: 4.4. Draw the specification limits on the distribution
Revision Date: 2005-09-06 Draw vertical lines on the distribution to represent the lower and upper specification limits. In the example, the lower specification limit (LSL) is 0 minutes (on time) and the upper specification limit (USL) is 14 minutes. Estimate where the two lines should be l ...
- ST: 2 What does it look like
Revision Date: 2005-09-06 A courier company has set up a team to look at the actual arrival time at customers’ locations to pick up packages, in comparison to the scheduled arrival time. The company guarantees pick up of packages within 14 minutes of the scheduled time. It is unacceptable t ...
- ST: 4.5. Calculate how much data is outside the specifications
Revision Date: 2005-09-07 As indicated in the previous step, some of the distribution is outside the specification limit. The question is, how much? To determine the percentage that falls outside the specification limits, it is necessary to find how many estimated standard deviations exist ...
- ST: 4.1. Sketch the distribution
Revision Date: 2005-09-06 Sketch a picture of a normal distribution. Begin by drawing a horizontal line (axis). Next, draw a normal (bell-shaped) curve centered on the horizontal axis. Then draw a vertical line from the horizontal axis through the center of the curve, cutting it in half. Th ...
- ST: 4.5.2. Find the percentage below the lower specification
Revision Date: 2005-09-07 The Z value for the lower specification is found by subtracting the lower specification from the overall average, and then dividing by the estimated standard deviation. The Z value for the lower specification is denoted as Zlower. The lower specification for the ...
- ST: 4.5.1. Find the percentage above the upper specification
Revision Date: 2005-09-07 The first step in determining the percentage above the upper specification is to calculate the Z value for the upper specification. This is found by subtracting the overall average from the upper specification, and then dividing by the estimated standard deviation ...
- ST: 4.5.3. Determining the total percentage outside the specifications
Revision Date: 2005-09-07 The total percentage outside the specification limits or requirements is found by adding the percentage outside the upper and lower specification limits. The total percent of output located outside the specification limits for the example is: 2.28 + 0 = 2.28% P ...
- ST: 4.3. Determine the location of the tails for the distribution
Revision Date: 2005-09-06 The next step is to determine where (at what value) the tails or ends of the curve are located. These values can be estimated by adding and subtracting three times the estimated standard deviation from the overall average. Remember, from the histogram section, that ...
- ST: 4.7. Analyze the Results
Revision Date: 2005-09-07 The completed analysis for the example is shown below. Calculations: = 2.00 Zupper = 2.00 Zlower = 5.00 Cpk = 0.67 Cp = 1.17 Cpu = 0.67 Cpl = 1.67 Examine the capability indices and the distribution. What do they show? Is the process capable? In the exampl ...
- ST: 4.6.1. Calculate Cp
Revision Date: 2005-09-07 Cp is an index used to assess the width of the process spread in comparison to the width of the specification. It is calculated by dividing the allowable spread by the actual spread. The allowable spread is the difference between the upper and lower specification l ...
- ST: 4.6.2. Calculate Cpk
Revision Date: 2005-09-07 Cpk takes into account the center of the data relative to the specifications, as well as the variation in the process. Cpk is simple to calculate. The smaller of the two Z values is selected. This is known as Z min . When Zmin has been selected, it is divided by 3 ...
