Nprocess capability cp cpk pdf

Cp and cpk are considered shortterm potential capability measures for a process. Cp and cpk are measures of the natural spread and centering of the population to specified limits. The statistics assume that the population of data values is normally distributed. In 1991, asq aiag task force published the statistical process control reference manual, which presented the calculations for capability indices cp, cpk as. Measuring your process capability symphony technologies. The cpk is an indicator of how centered your process is use cp and cpk together to evaluate this. Process capability studies the following is based in part on the paper process capability studies prepared by m. Pharmaceutical6pharmaceutical6sigmaqualitybydesignqualitybydesignqualitybydesignqualitybydesignsigmaajazs. The six sigma handbook revised and expanded thomas. Note that cpk is closely related to cp, the difference between cpk and cp represents the potential gain to be had from centering the process. Process capability analysis and process analytical technology. The six sigma handbook revised and expanded thomas pyzdek. Understanding process capability indices statistics and actuarial. Process capability cp, cpk and process performance pp.

A control chart always has a central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. The cpk calculation assumes that the data is normally distributed. Analyze covers establishing a process base line, how to determine process improvement goals, knowledge discovery, including descriptive and exploratory data analysis and data mining tools, the basic principles of statistical process control spc, specialized control charts, process capability analysis, correlation and regression analysis. Cp, cpl, cpu, and cpk are the four most common and timed tested measures of process capability. Minimum criteria educationhigh school or equivalent. Process capability indices measure the degree to which your process produces output that meets the customers specification.

Cp is an index of the spread of the population in relation to the spread of the spec limits. Pdf process capability and capability index cp and cpk. Process capability cp is a statistical measurement of a process s ability to produce parts within specified limits on a consistent basis it gives us an idea of the width of the bell curve. Variability can be stated as either shortterm or longterm. In both cases we want to try to verify if the process can meet to meet customer ctqs requirements. The main difference between pp and cp on one side and ppk and cpk on the other side is whether we use complete set of data for calculation pp and ppk where we calculate real performance of the system, or we use sample preproduction, batch, logical subgroups where we calculate capability of the process. Control chart statistical process control charts asq. The control chart is a graph used to study how a process changes over time. The process capability for a stable process, typically defined as usllsl 6 x standard deviation. The process capability requirements given in section 4. Cp and cpk are based on short term variability pp and ppk are based on total variability.

Work experienceat least 3 years of business, technical, or managerial experience. Measures of process capability process capability indices. A process capability index is an aggregate measure of a processs ability to meet specification limits. Figure 3 illustrates the effects of process capability due to shifting in the mean with variation remaining the same. Cp is process capability, it compares the process capability to the maximum allowable variation as indicated by the process. Purpose separate common cause from special cause variation communicates process performance over time.

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