the data should be tested for normality.

the sample size must be adequate (large enough).

The following conditions must be met before Cp and CpK can be successfully used to evaluate the ability of a process: The index indicates whether the process is capable of producing within specification and is also an indicator of the ability of the process to adhere to the target specification. This Cpk index is calculated using specification limits, the standard deviation, and the mean. No information on the ability of the process to adhere to the target value is included in this index. This index indicates, in general, whether the process is capable of producing products to specifications. The Cp index is calculated using specification limits and the standard deviation only. Mean The arithmetic average of a group of values. The standard deviation is normally fixed for a process that is under statistical control and can only be affected by a process change that affects the variability in a process.

Defined as the root mean square (RMS) deviation from average it indicates how much a process can be expected to vary from the average. Standard Deviation The standard deviation is a measure of variability in a process. Similarly the lower specification limit (LSL) is the lower limit of the specified range. The upper specification limit (USL) is the upper limit of the specified range. A specification for a piston ring, for example, might specify that the diameter be 74mm +/- 0.05mm. Specifications are normally defined in terms of nominal (+/-) tolerances or ranges (low to high. In other words, they define what is expected from an item for it to be usable. Specification Specifications define product requirements.

Specifications: Specifications define product requirements.To understand Cp and Cpk you must have an understanding of the following terms: These indices, which are a fairly recent addition to the field of statistical process management, greatly simplify the management of statistically controlled processes. Cp and Cpk, commonly referred to as process capability indices, are used to define the ability of a process to produce a product that meets requirements.

Cp and cpk calculations in excel