Process capability cp, cpk and process performance pp. The lower and upper engineering specification limits are 1. Measuring your process capability symphony technologies. Process capability determines if a process will deliver a product or service that meets the customers requirements. Process capability and process control how are they. Introduction a process capability index is a numerical summary that compares the behavior of a product. Process capability is one method of measuring the effectiveness of a process in meeting standards or customer specifications as well as measuring process improvement efforts. Your process should be stable and the original or transformed process data should follow a normal distribution.
Cp cpk capability indices pp ppk performance indices use with a sample when testing the capability of a process. Liptak process measurement and analysis documents pdf finder. Capability analysis overview capability analysis is used to evaluate whether a process is capable of producing output that meets customer requirements. Process capability analysisis defined as the engineering study to calculate the process capability. The process capability and performance analysis has become an inevitable step in quality management of modern industrial processes. In the absence of any special or assignable causes of variation, a process will still have some inherent variability. Generally you use this when a process is under statistical control. Estimating quality presents a systematic exploration of process capability analysis and how it may be used to estimate quality. When a point is outside these established control limits it indicates that the mean of the. You cant perform a process capability analysis unless your process is in statistical control. The capability of a process is defined as the ratio of the distance from the process center to the nearest specification limit divided by a measure of the process variability. In a later modules we will discuss capability analysis. To assure valid results when performing a capability analysis, follow these steps. Process capability is defined as the minimum spread of a specific measurement variation which will include 99.
Process capability compares the output of an incontrol process to the specification limits by using capability indices. Process capability studies are best conducted on processes that have reduced. Once the control limits have been established of the x bar charts, these limits may be used to monitor the mean of the process. One basic measure of the capability of the process is the proportion of values falling inside or outside the specification limits. Assumptions there are two critical assumptions to consider when performing process capability analyses with continuous data, namely. The histogram and fitted normal curves provide an informal way to quickly check for. Process capability and six sigma methodology including fuzzy. There are two critical assumptions to consider when performing process capability analyses with continuous data, namely.
Process capability cp cpk study with excel template. Below, within the steps of a process capability analysis, we discuss how to determine stability and if a data set is normally distributed. Ppk is a short term analysis mostly done with 50 or 100 samples measured ppk is process potential capability cpk is long term analysis and is continous process capability. Customer requirements are defined using an upper specification limit usl and a lower specification limit lsl. Mario perezwilson mpcps original methodology for process. Process capability is the longterm performance level of the process after it has been brought under statistical control. Control chart and process capability analysis in quality. The capability of a process is a statistical indicator of how well it is functioning, or, in. Process capability analysis o ur focus for the prior publications in this series has been on introducing you to statistical process control spcwhat it is, how and why it works, and how to use various tools to determine where to focus initial efforts to use spc in your company. Process capability study is a scientific and a systematic procedure that uses control charts to detect and eliminate the unnatural causes of variation until a state of statistical control is reached.
Process capability by sanjaya kumar saxena the capability of a process is a statistical indicator of how well it is functioning, or, in other words, how successful it is at running within its specified limits. This article is devoted to the topic of process capability, with the objective of making people aware of this. The objective of the study was to estimate the current process. Abstract the robustness of two popular process capability ratios, cp and cpk, when the random process being observed departs from normality is analysed. The idea is illustrated graphically in figure 1 that shows a histogram of the process.
Procedures when dealing with nonnormal data will be considered in the following edition of extraordinary sense. T o some unique combination of machines, tools, methods. This process will produce conforming products as long as it remains in statistical control. Here we discuss the following topics relating to process capability. Interpret the key results for normal capability analysis. Computes process capability indices for a qcc object of type xbar and plot the histogram. A process capability study is a systematic procedure for determining the capability of a process. Process capabilitya managers tool for 6 sigma quality.
Process capability analysis and process analytical technology. Pdf the process capability analysis a tool for process. T ake representative sample of process output statistical analysis of samples ma thematical tools, scattered plot, pareto chart. It is a statistical tool to measure the variability of the process and it is a part of statistical process control.
A capability index relates the engineering specification determined by the customer to the observed behaviour of the process. Process capability is the measured, inherent variation of the p roduct turned out by a process what is process. Process capability analysis march 20, 2012 andrea spano andrea. Process capability is a statistical measurement of a process s ability to manufacture parts within the specification consistently. Statistics definitions process capability analysis is a set of tools. The distribution of the process considered is normal. Process capability index example a manufacturing process produces a certain part with a mean diameter of 2 inches and a standard deviation of 0.
Process capability assesses the relationship between natural variation of a process and design specifications an indication of process performance with respect to upper and lower design specifications application of process capability design products that can be manufactured with existing resources identify process. One basic measure of the capability of the process. Review of the development in process capability analysis. Process capability analysis for three stages of the process, generally three stages of the process has been able to meet the specified specifications. Pdf process capability and capability index cp and cpk. This process should produce less than 64 nonconforming ppm. In both cases we want to try to verify if the process can meet to meet customer ctqs requirements. Based on the process capability analysis one can determine how the process performs relative to its product requirements or specifications. Process capability and six sigma methodology including. The comparison is made by forming the ratio of the spread between the process specifications the specification width to the spread of the process values, as measured by 6 process standard deviation units the process width. An important part within process capability analysis is the use of process capability indices. When the relevant measure of performance is a rate, then the process capability analysis is based on. Learn all 4 types of process capability indices cpk, cp, ppk.
Process capability can be expressed as percent nonconforming or in terms of the natural spread related to the specification spread. Once the control limits have been established of the x bar charts, these limits may be used to monitor the mean of the process going forward. Process capability analysis for normal distributions cp and ppm interpretation process spread lsl usl specifications. Let we understand this concept with the help of a simple example. Capability is defined as the ability of a process to produce outputs that meet the product service and or the customer specifications. Process capability analysis for six sigma 6 6 length frequency 5. In a later modules we will discuss capability analysis for nonnormal data. Frequently, quality practitioners find that the assumption of modeling a process using the normal distribution is not valid. In the absence of any special or assignable causes of variation, a process. To this pall of darkness, mario perezwilson, with his book on machine process capability study, brings a beacon of light. Process capability cp, cpk and process performance pp, ppk. Automotive industrie requires a ppk better than 1,67 and cpk better than 1,33 or ppk better than 2,0 and cpk better than 1,67 f. 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. He carefully orchestrates a stepbystep methodology process delineation.
It is the ability of the combination of your 5 ms to produce a product that. Process capability analysis for six sigma 2 chapter highlights this chapter deals with the concepts and applications of process capability analysis in six sigma. The minitab assistant includes two capability analyses to examine continuous process data. Process capability is a measure obtained by taking a representative sample of process output, performing a statistical analysis and using the results obtained to determine future expected process yields.
Any whitepaper or book can be customized to reflect a companys improvement process. Capability analysis capability analysis, or process capability analysis, is the comparison of the distribution of sample values to the specification limits, and possibly also the specification target. Process capability is a statistical measure of this inherent. Capability analysis of complex parts pdf the background and implications of the military aircraft and missile systems group developing appropriate methods for monitoring processes and evaluating process capability. The output of a process is expected to meet specifications, which can be determined according to the cu stomer requirements.
Process capability studies are done on the machines to find out the capability of individual mcs. It is a statistical tool to measure the variability of the process and it is a part of statistical process. Process capability analysis for nonnormal processes with. Process capability will provide a single number which details the ability of a process. In other wordsprocess capability analysis helps to estimate, monitor, and reduce the variability in the processes. Your process should be stable and the original or transformed process. If these assumptions are not met, the resulting statistics may be highly unreliable. Chapter 1 process capability analysis in this chapter we give the mathematical background of process capability analysis, in particular the capability indices cp and cpk and related topics like tolerance intervals and density estimation. Process capability an overview sciencedirect topics.
Use with the total population when testing the performance of a process. Assessing process capability section adapted from whats meant by capability. The comparison is made by forming the ratio of the spread between the process specifications the specification width to the spread of the process values, as measured by 6 process standard deviation units the process. Frequently, quality practitioners find that the assumption of modeling a process. Process capability is usually measured using either the capability ratio cp or the capability index cpk. Key output includes the histogram, normal curves, and capability indices. A capable process is a process whose spread on the bellshaped curve is narrower than the tolerance rangecustomer specifications. Apr 20, 2019 process capability is a statistical measurement of a processs ability to manufacture parts within the specification consistently. Pca is a prominent technique that is used to determine how well a process meets to these specification limits.
Generally you use this when a process is too new to determine if it is under statistical control. Process capability analysis using minitab ii by keith m. Process capability uses the process sigma value determined from either the moving range, range or sigma control charts pp and ppk are used for process performance. Where the chance causes of variation are compared to the engineering specifications. Process capability assesses the relationship between natural variation of a process and design specifications an indication of process performance with respect to upper and lower design specifications application of process capability design products that can be manufactured with existing resources identify process weaknesses. Pdf spc and process capability analysis case study. Process capability analysis o ur focus for the prior publications in this series has been on introducing you to statistical process control spcwhat it is, how and why it. Process capabilitya managers tool for 6 sigma quality advantage. The determination of process capability requires a predictable pattern of statistically stable behavior most frequently a bellshaped curve. Process capability is a measure of the inherent process performance. Pdf process capability can be evaluated through the computations of various process capability ratios and indices. Understanding process capability indices stefan steiner, bovas abraham and jock mackay institute for improvement of quality and productivity department of statistics and actuarial science university of waterloo waterloo, ontario n2l 3g1 1.
Abstract this paper builds on the content covered in the previous edition of extraordinary sense. You cant perform a process capability analysis unless your process. The book is designed for practitioners who are tasked with insuring a high level of quality for the products and services offered by their organizations. Process capability analysis is an important part of an overall quality improvement program. Complete the following steps to interpret a normal capability analysis. For information, call, write, or email to the address above.