The Relationship Between Cp/Cpk and Sigma Level, Use of Six Sigma Tools with Discrete Attribute Data (Pass/Fail)/FMEA, How to Write an Effective Problem Statement, High-performance Teams: Understanding Team Cohesiveness, Preparing to Measure Process Work with a Time Study, The Importance of Implementing Effective Metrics, The Implementation Plan – Getting Beyond the Quick Fix, Lean Six Sigma and the Art of Integration, Most Practical DOE Explained (with Template), Director, Process and Compliance - Spectrum Enterprise, Using the Power of the Test for Good Hypothesis Testing, Six Sigma Aids in Resource Planning for IT Employees, Best Practices for Process Maps at California High-Speed Rail Authority, Quick Wins Can Successfully Launch Operational Excellence in Healthcare, Using Critical Path Analysis to Prioritize Projects, Why You Cannot Depend Totally on Statistical Software, Case Study: Streamlining Coast Guard's Accounts Payable Process, Case Study: Reducing Delays in the Cardiac Cath Lab. Try using the following analogy to explain the subject more easily.eval(ez_write_tag([[320,50],'isixsigma_com-medrectangle-3','ezslot_1',181,'0','0']));eval(ez_write_tag([[320,50],'isixsigma_com-medrectangle-3','ezslot_2',181,'0','1'])); Imagine you are driving on a highway that is undergoing construction. iSixSigma is your go-to Lean and Six Sigma resource for essential information and how-to knowledge. Process Capability Indices A process capability index uses both the process variability and the process specifications to determine whether the process is "capable" We are often required to compare the output of a stable process with the process specifications and make a statement about how well the process meets specification. Not many customers are willing to widen the specification limits; they are what you agreed to initially. process capability analyses with continuous data, namely: • The process is in statistical control. This prediction enables us to “qualify" a new manufacturing process as being fit for use in production. A process that is capable indicates the car (process) is skinnier than the garage you’re trying to park it in (the specification width). These “explanations” only illustrate graphically and analogously the formulas–not the meanings. If we look at a simple example of making a cup of coffee, we can identify the inputs, steps, equipment and output of the process. All capability indices are just probabilities of meeting specifications. Process Capability Analysis and Process Analytical Technology Presented by: Steven Walfish President, Statistical Outsourcing Services steven@statisticaloutsourcingservices It’s clear now why the 18-wheeler will be accompanied by a vehicle with warning signs and lights on it – its job is not only to warn oncoming traffic, but also to help identify potential hazardous situations (like the barriers). To better represent the population values, the sample data must be randomly collected, preferably over time from a large production run. When we look at the bigger picture, process capability is more than just measuring Cp and Cpk values. Quality-One offers Quality and Reliability Support for Product and Process Development through Consulting, Training and Project Support. a process performance analysis should also be performed during Stage II. Lower Ppk values indicate that your process may need improvement. the distance from the center of your vehicle to the closest barrier, divided by the distance from the center of your vehicle to the edge of the vehicle closest to that same barrier. Important knowledge is obtained through focusing on the capability of process. You are correct. The Cpk value should be between 1 and 3. The Pp and Ppk indices are used to evaluate a new process or one that is not in statistical control. Any special causes must be discovered and resolved. With the 18-wheeler you do not have any wiggle room; the manufactured home would be ruined if you attempted to go through the barriers – intentionally or accidentally. Some of the inputs are coffee and water. And remember to center your processes, reduce your variation and increase your process capabilities. In this situation a motorcycle running 3 feet away from the barrier would have a capability of 2.0 while the compact car would have a capability of 1.0. Join 60,000+ other smart change agents and insiders on our weekly newsletter, read by corporate change leaders of: 50 Percent Sampling Savings with Sequential Test Method, Resolving Common Issues with Performance Indices, How to Avoid Common Mistakes in Linear Regression. It’s as if you “explained” Centigrade by telling me the formula to convert from Fahrenheit but never told me it was a temperature metric. What is Process Capability Index (Cpk)? As a customer it can be argued that it makes more sense to know what the process performance is as opposed to what the process is capable off, i.e. In addition, by knowing your process capabilities, the design team can work with manufacturing to improve product quality, and processes that are “not in control” may be targeted for improvement. The primary aims of an employer's capability procedure should be to: 1. provide a clear framework setting out required performance standards; 2. encourage improvement where necessary and address any root causes of poor performance; 3. establish a mechanism for dealing with any initial performance concerns on an informal basis; 4. have in place a formal procedure for dealing with any significant performance concerns or where previous informal discussions have not led to the required improvement. 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. Process Capability analysis Process: - T ake representative sample of process output - Statistical analysis of samples (Ma thematical tools, Scattered Plot, Pareto Chart, Also, the statement of why to use Cpk is because we can only get damaged on the closest side implies you can’t get defects on the other side. Part 2 looks at failure rates and material conditions. A process where almost all the measurements fall inside the specification limits is deemed a capable process. A process has been defined as a sequence of interdependent procedures, operations or steps that consume resources and convert the inputs into outputs. I always tell my students to look at a histogram with the specs shown on it if they don’t have good statistical software like Minitab which gives them the Cp and Cpk. First, don’t get these guys judging Cp without looking at Cpk also. Using the values previously noted, the vehicle capabilities are: Which vehicle would you prefer to drive? [Potential] Process Capability Analysis (Cp, Cpk): A process capability study uses data from a sample to PREDICT the ability of a manufacturing process to produce parts conforming to specifications. Would you try to go the speed limit through these barriers or would you slow down? A good analogy is parking a car in a garage. The one with the highest capability – the motorcycle.eval(ez_write_tag([[320,50],'isixsigma_com-large-leaderboard-2','ezslot_12',182,'0','0']));eval(ez_write_tag([[320,50],'isixsigma_com-large-leaderboard-2','ezslot_13',182,'0','1'])); Now consider a variation. The motorcycle I have described still has the same Cp as your motorcycle from Monday but is entirely out of specification (when taking into the driving rules of the road). Overall capability indicates the actual performance of your process that your customer experiences over time. In this video, I explain the concepts of process capability and process capability index. The data should be randomly collected from a large production run. What you have experienced is varying degrees of capability, ranging from good capability (the motorcycle with room to roam inside the lane) to poor capability (18-wheeler towing a manufactured home taking up more room than is available). This is Part 1 of a two-part article on process capability. We say that a process is capable if virtually all of the possible variable values fall within the specification limits. Now let us examine the Process capability report: Using the graph, we can further evaluate process capability by comparing the spread or range of the product specifications to the spread of the process data, as measured by Six Sigma (process standard deviation units). Monitoring process capability allows the manufacturing process performance to be evaluated and adjusted as needed to assure products meet the design or customer’s requirements. Have you tried to explain the concept of process capability to someone who then looks at you like you have two heads? The standard process capability evaluation is based upon the assumption that the process under consideration is Normally distributed, as the assumption that x±3σ covers 99.73% of the area under the process curve only applies for Normal distributions. The variation can occur in the amount of coffee or water introduced in the process and the performance of the coffee maker itself. (248) 280-4800 | information@quality-one.com, FMEA – September, 2020 – Virtual Workshop, Core Tools – September, 2020 – Virtual Workshop, 8D/RCA – September, 2020 – Virtual Workshop. Is “Statistical Significance” Outdated? Each operation or step adds to the next to achieve a goal or desired result. IMO, it’s a significant omission. For our example, we will utilize data from randomly collected measurements of a key characteristic of a machined part. set of calculations used to assess whether a system is statistically able to meet a set of specifications or requirements Use Ppk to evaluate the overall capability of your process based on both the process location and the process spread. On this day you are driving a motorcycle. To learn more about what is Process Capability Index (Cpk), continue on! But to a common man, Cpk value does not make any sense. We can maintain Process Capability. Reduce your variation so that your process fits between the existing barriers. I’m sorry but you have to look at the other side. The lines defining your driving lane are your specification limits. USL (Upper Specification Limit) = 25.527 mm, LSL (Lower Specification Limit) = 25.273 mm. As you approach the barrier, your opportunity for success is lowered. Consider the warning vehicle to be a company’s quality department. To illustrate, let us use a real world example. Process Capability - Cp, Cpk. The process capability index is used to check the variability of the output generated by the process and to compare the variablity with the product tolerance. If the process behaves consistently over time, then we say that the process is stable or in control. Process Capability means that no matter how wide you are most of the time, you’ll make it sometimes … perhaps after many, many tries. This is equivalent to what is typically called Cp – your process potential – calculated as the width of your specification (the vehicle) divided by your variability (the barrier). Accurate knowledge of process capability enables management to make decisions regarding where to apply available resources based on data. Hi, this is Mike Negami, Lean Sigma Black Belt. All the data points fall well within the specification limits with a normal distribution. Accurate process capability studies are dependent upon three basic assumptions regarding the data: In manufacturing and many other types of businesses, reduction of waste and providing a quality product are imperative if they are to survive and thrive in today’s marketplace. The specification is the barrier (10 feet) / Vehicle Process (14 feet) = .71 Cp. Imagine that you are driving your vehicle over a bridge. On Wednesday you hop into an 18-wheeler for the same trip. Generally, higher Ppk values indicate a more capable process. Capability is defined as the ability of a process to produce outputs that meet the product / service and / or the customer specifications. The Cp index is a fundamental indication of process capability. The manufactured home industry has figured out how to get around this dilemma – they ship the homes in sections, literally cutting their variation in half. Process Stability refers to the consistency of the process with respect to important process characteristics such as the average value of a key dimension or the variation in that key dimension. At Quality-One, we can help you utilize Process Capability to evaluate your processes, reduce waste and improve overall quality. You are greeted by one narrow lane with modular concrete barriers on both sides. The Cpk index of process center goes a step further by examining how close a process is performing to the specification limits considering the common process variation. During a typical Kaizen event or other quality improvement initiatives, Process Capability is calculated at the start and end of the study to measure the level of improvement achieved. If you do not stay centered in your lane you run the risk of causing damage to you and your vehicle. In process speak, if you are filling your tolerance zone with process variation even when your process is centered, might it also be a good idea to slow your process down? We are honored to serve the largest community of process improvement professionals in the world. The concept of process capability only holds meaning for processes that are in a state of statistical control. Generally, higher Ppk values indicate a more capable process. What shoul… For the example in this section, we will utilize a popular statistical software package. If your process has a Cp of less than 1, even if your process is centered, you will make bad product. The vehicles are average widths: All the vehicles are running straight down through the lane – your process is centered on nominal (the target value). In process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability: the ability of a process to produce output within specification limits. There are cases when the data does not fit a normal distribution. Driving in the middle of your lane you have plenty of room on either side. The process capability is a measurable property of a process to the specification, expressed as a process capability index (e.g., C pk or C pm) or as a process performance index (e.g., P pk or P pm). The larger the Cpk value the closer the mean of the data is to the target value. You are extremely nervous now – not only do you have almost no room on either side of your vehicle but you also must tow something that takes up more room than the entire driving lane. The Cp and Cpk calculations use sample deviation or deviation mean within rational subgroups. The Pp and Ppk calculations use standard deviation based on studied data (whole population). Process capability analysis is the determining if a process can meet specification.Here we want to use Pp for new processes or Cp for processes that are under statistical control. Cpis governed by following formula: John, I’ve seen variations of this explanation (e.g., my clients’ Six Sigma materials). That leaves only two options: Now you should be able to explain process capability to anyone who is new to the concept. One consideration is that certainty (the fixed width of a truck) is not the same as probability or variance (how likely is it that I will be that “wide”). This time you do not have as much room between the barriers – only a couple of feet on either side of the vehicle. The Cp value is calculated using the specification limits and the standard deviation of the process. In every process, there exists a certain amount of variation. Assume that the modular concrete barriers around you are 10 feet wide. Six Sigma – Process Capability Explained Quality Improvement Solutions & Statistics for Business Management, Personal Improvement, Education, Quality Engineering, and SC Manufacturing Lean Six Sigma: How to Conduct a Lean Event – by Quality Minds, Inc | Charleston SC Human Capital Firm Today, we’ll discuss the basics of Process Capability, Cp, Cpk and Unilateral (one-sided) Tolerance. There are no special causes of variation in the process and it is in a state of statistical control. Implementing SPC involves collecting and analyzing data to understand the statistical performance of the process and identifying the causes of variation within. Just when you are feeling better about getting through Wednesday’s drive, you are asked to drive the same 18-wheeler, but this time you will be towing a manufactured home behind you! Today you have to find an alternate route. When engineers are designing parts, they must consider the capability of the machine or process selected to produce the part. • If these assumptions are not met, the resulting statistics may be • In a later • The distribution of the process considered is Normal. Increase familiarity with process capability, enable a process improvement mind-set, and socialize the process-capability model Assess the efficiency and effectiveness of process-capability program Develop a consistent approach for calculating process-capability indices, and conduct a systematic assessment to measure and compare process-capability maturity within and across organizations You must keep your vehicle on the bridge to reach the other side. 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 I’ve used similar examples when teaching process capability. Process capability index (Cpk) is a statistical tool, to measure the ability of a process to produce output within customer’s specification limits. At Quality-One, your success is our business! You wrote: All the vehicles are running straight down through the lane – your process is centered on nominal (the target value). You do not want to be that close – you may be scraping the paint off the side of your car. Many companies require at least 25 to preferably 50 sample measurements be collected. In simple words, it measures producer’s capability to produce a product within customer’s tolerance range. This is equivalent to what is typically called Cp – your process potential – calculated as the width of your specification (the vehicle) divided by your variability (the barrier). You are outside the lane (out of specification) before you start to drive. Variation in a process cannot be eliminated, but it can be measured, monitored, reduced and controlled. John Evelyn • It’s a very nice first step, but I respectfully suggest maybe not a place to leave the customer. Whether you’re interested in Process Capability Consulting to assist in planning and implementation, Process Capability Training to bring your team up to speed or Process Capability Support to help you drive process improvement using Process Capability studies or other SPC methodology, we can provide the help you need, enabling you to accomplish your process improvement goals. Process Capability means that no matter how wide you are most of the time, you’ll make it sometimes … perhaps after many, many tries. Should that not be “the width of your specification (the barrier) divided by your variability (the vehicle)”? Process capability is just one tool in the Statistical Process Control (SPC) toolbox. Process Capability (Cp) is a statistical measurement of a process’s ability to produce parts within specified limits on a consistent basis. Each operation or step adds to the next to achieve a goal or desired result. July 12, 2013 at 9:27 am - Log in to Reply Hoda El Shibiny This is not a problem, but you do have to be a bit more careful of going into and beyond the barriers or, in process capability speak, out of specification.eval(ez_write_tag([[320,50],'isixsigma_com-box-4','ezslot_4',139,'0','0']));eval(ez_write_tag([[320,50],'isixsigma_com-box-4','ezslot_5',139,'0','1'])); Now the fun begins. When we say the Cpk of the process is 1.22, what meaning does i 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. The sample data is representative of the population. This time you have hardly any room on either side of your vehicle. Part 1 addresses the concept of process capability and how to calculate it, including what to do with data that is normal and non-normal. Most companies require that the process Cp = 1.33 or greater. We help businesses of all sizes operate more efficiently and delight customers by delivering defect-free products and services. As previously mentioned the information gained is generally used to reduce waste and improve product quality. The output of this measurement is often illustrated by a histogram and calculations that predict how many parts will be produced out of specification (OOS). The reason why you only use the closest side is that this is the only one in danger of being damaged. The graphic on the left below illustrates a stable process. Cpk is calculated using the specification limits, standard deviation or sigma, and the mean value. In this example, you are running your vehicle (or process) off center, thereby getting closer to the barrier (specification limit) on one side. If a process capability analysis is performed during Phase I then this fact must be clearly documented and the reasons and limitations explained. The width of your vehicle is equivalent to the spread or range of the data. When used effectively this information can reduce scrap, improve product quality and consistency and lower the cost to manufacture and the cost of poor quality. Cpk is a capacity record that tells how well a framework can meet specification limits. Well within the specification limits ; they are what you agreed to.. 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