process capability minitab interpretation

Process Capability Index – Cpk: This is the capability index that accounts for the centering of the process and is defined as the minimum of Cpk upper and Cpk lower. Stat Control Charts 2. Calculation and proper interpretation of process capability indices is based on following suppositions: data distribution can be approximated by the normal distribution A process is a unique combination of tools, materials, methods, and people engaged in producing a measurable output; for example a manufacturing line for machine parts. The CM index is generally used to evaluate machine capability studies, rather than full-blown process capability studies. Compare Ppk and Cpk. Learn real-life applications for the most commonly used graphs and statistical tools, including Hypothesis Tests, Control Charts, Process Capability, ANOVA, Regression and more. When a process is in statistical control, Ppk and Cpk are approximately equal. So we sample 32x and we can calculate the cpk of the dimension from the 32x data. Figure 3. These statistics tell us how well the process is meeting specifications or requirements. following section. Interpreting a Process Capability Chart. When we talking about the capability of a process , we usually use cpk to show how well the process is. In this case the process variation is measured by 6 standard deviations (+/- 3 on each side of the mean). 2. Six Sigma process performance is reported in terms of Sigma. According to Six Sigma philosophy, Cp or Pp and Cpk or Ppk should be greater than 1.50. Computer software (Minitab, Statgraphics, Statistica, QI Analysis, and more) calculates these as part of their capability analysis. 8 (P8) Capability for One Spec Limit. 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). Both of these tools are easy to use in Minitab, but you first need to properly setup your data. We interpret the results of a non-normal capability analysis just as we do an analysis done on data with a normal distribution. The capability for a p-chart is the average proportion of nonconforming items (p-bar). Capability analysis for attributes. The histogram and the normality test results appear in the new window. Since varia tion will increase when normal sources of process variation are added (e.g., tooling, fixtures, materials, etc. Minitab is a popular tool that allows one to perform various statistical calculations such as normality test , process capability analysis … A capable process is a process whose spread on the bell-shaped curve is narrower than the tolerance range/customer specifications. Process capability is defined as a statistical measure of the inherent process variability of a given characteristic. 3. A capability index relates the engineering specification (determined by the customer) to the observed behaviour of the process. Capability studies are designed for two-sided specifications that have an upper and a lower limit. Cp tells you if your process is capable of making parts within specifications and Cpk tells you if your process is centered between the specification limits. The Capability Plot is one of six charts created by QI Macros Capability Suite. But the statistical measurements of Cp, Cpk, Pp, and Ppk may provide more insight into the process. Attributes capability measures are taken directly from attributes control charts. To assess and quantify how well your process falls within specification limits, you can use capability analysis. U-Chart (Minitab Case Study) Process capability – Cp : This is defined as the tolerance width divided by the total spread of process (6 Sigma). Click “OK.”. Cpk and Ppk indicate process capability considering both variability and … Visualizations are good, but pair them with analytics to make them great. Step-2: let’s get started with an Example, A sheet metal manufacturing company was producing Brake pad. Implement statistical process control methods in production. Decision of process capability analysis. Process capability indices Cp and Cpk evaluate the output of a process in comparison to the specification limits determined by the target value and the tolerance range. Cpk is calculated from the sigma using R-bar/d2. For example , if a dimension is a key characteristic of a product , we have USL and LSL from the drawing. Find the last positive point. Section 5: Minitab Case Study. Capability Analysis ... Two ways to create control charts in Minitab 1. In Minitab, each column in the worksheet typically represents a different variable. Steps in Minitab to run a process capability analysis: Click Stat → Basic Statistics → Normality Test. Minitab Process Capability Tutorial (Step by Step guide): Step-1: When you will open the Minitab 18, the main screen will appear like below. Process capability is estimated by calculating, the so called process capability indices. ... process shifts Interpretation: If a point is above the UCL, scan to the left of that point. Figure 3 contains a lot of information useful for a PCA assessment. Minitab Statistical Software can look at current and past data to find trends and predict patterns, uncover hidden relationships between variables, visualize data interactions and identify important factors to answer even the most challenging of questions. This is where the process shift began. Learn the definitions, interpretations and calculations for Cp, Cpk, Pp and Ppk. All terms and concepts used in Capability Analysis. normal distribution curve used to approximate the process. Demonstrate understanding of the important relationship between capability analysis and process stability, as observed on control charts. A logistics manager for an online store that has more than 1,000 distribution centers wants to assess the sales of the centers by region. Process capability analysis is a major concept and a must study for professionals looking forward having a firm knowledge on Minitab with Statistics. Each bar on the histogram represents the frequency of data within an interval. Once statistical process control is evidenced, then the histogram and process capability may be analyzed. 1. Since process capability is not valid unless the process is stable, always look at a control chart of the data first. Process Capability Studies Using One-Sided Specifications. Between/Within Capability Analysis. The difference between Ppk and Cpk represents the improvement in process capability that you could expect if shifts and drifts in the process were eliminated. Concept of Process Capability First introduced in Statistical Quality Control Handbook by the Western Electric Company (1956). Here’s how. Construct and interpret control charts for variables and attributes. Ppk is Calculated from the sigma using (n-1). Capability is defined as the ability of a process to produce outputs that meet the product / service and / or the customer specifications. Cp and Pp indicate the capability of the process based only on its variability. The wire was rotated 22.5° after each shot; rotating a weak and bent wire simulated a cause of variation in the process. Cp indicates the spread of variation present in a process. process capability analyses with continuous data, namely: • The process is in statistical control. Process Capability Study Steps •Four Basic Steps 1. Cpm is calculated from the sigma using (n-1), and it is based off a Taget Value instead of the mean of the tolerance. The attached Capability Study Form may be used for: - … • The distribution of the process considered is Normal. Create the Same Charts as Minitab's Capability Sixpack™ for a Fraction of the Cost. It means sample spread width. Process Capability measures how consistently a manufacturing process can produce parts within specifications. 12 (P12) Common Type of Problem in Process Capability. Chapter 6: Process Capability Analysis for Six Sigma 6 6 Length Frequency 5.96 5.98 6.00 6.02 6.04 30 25 20 15 10 5 0 5.95 5.999 6.05 Mean 5.999 StDev 0.01990 Below is an explanation of what the 3 lines represent. The basic idea is very simple. Minitab is the world's leading statistical software for Six Sigma and statistics education. Minitab, JMP, and Matlab software provide support for Process Capability Analysis. ), C M uses a four sigma spread rather than a three sigma spread. The length of the Brake pad was 150±1mm. Process capability studies calculate the process capability indices Cp and Cpk. 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. to the width of the specification (VOC Voice of the Customer.We would like the process spread to be smaller than, and contained within, the specification spread. Recommend process improvements that increase process capability Finally, you will learn how to implement your optimization measures on the process more efficiently in regard to time. The value Cp is generally gives idea about the width of bell curve. The Minitab Capability Sixpack results for the run using a thin wire are depicted below. You can use a process-capability study to assess the ability of a process to meet specifications. The capability histogram shows the distribution of your sample data. Calculate capability indices and determine the level of quality 4. 1. (P7) Process Capability Index Cpk. As you can see in the above figure, we have considered the three conditions as Cp=1, Cp>1 and Cp<1. Quality Trainer contains more than 250 interactive lessons you can use and repeat at your own pace. We want to know the capability of the process . On a final note, some situations do occur where specifications such as 10 +0.1 / -0 actually convey a target (ideal) of 10 instead of the usual "halfway between the limits" interpretation. A capability plot is just another way to visualize the capability of a process. A new window named “Normality Test” pops up. There are three comparisons made from these results: % of overall variation (StudyVar), % of Tolerance (Spec Limit width, if entered into report), and % of Contribution. Interpretation of the C p index: This equation just says that the measure of our process capability is how much of our observed process variation is covered by the process specifications. If you want to perform capability analysis on each of the variables contained in several different columns without having to run a separate analysis for each one, you can use the following procedures. You want your manufacturing process to: (1) be centered over the Nominal desired by the design engineer, and. –process capability is defined as the natural or undisturbed performance after extraneous influences are eliminated. The first chart (upper left corner) is your breakdown of variation, into Part, Repeatability, and Reproducibility. Verify that the distribution of the data is normal 3. This is determined by plotting data on a control chart. Key Result: Ppk. ex. • In a later modules we will discuss capability … In the figure above, sample 20 is an out of control point. For these process data, Ppk = 0.52. As capability analysis is covered, the course then introduces to you all the main optimization techniques for improving the process capability through 7 main steps within the optimization algorithm. 11 (P11)- Capability Study. Minitab was chosen for an example. Use Minitab software for data analysis and identifying trends. Process Capability can be evaluated through the computations of various process capability ratios and indices. Six Sigma Process Capability & Performance Limits. It lets you answer the question "is the process that you see today going to be similar to the process that you see tomorrow?" Minitab has an option to specify a limit as a boundary which should be enabled in this case. We will see in graph that width of the bell curve is within specification limit or not. 1. It consists of the following tools with practical examples for easy understanding and better clarity. Process Capability Index: Cp and Cpk. No additional calculations are required. Minitab is general-purpose statistical software package that provides a nice graphical summary of process capability, figure 3. Select “HtBk” as the variable. Process Capability Analysis for Normal Distributions CP and PPM interpretation Process Spread LSL USL Specifications Case 2: 1 < CP < 1.33 A barely capable process This process will produce greater than 64 From a technical standpoint, Six Sigma deems a process being acceptable only after achieving a maximum defect rate of 3.4 parts per million opportunities. Process Capability analysis Process: - T ake representative sample of process output. • If these assumptions are not met, the resulting statistics may be highly unreliable. 9 (P9) Long Term Capability Study (Pp and Ppk) 10 (P10) Acceptance Criteria -Capability Study. (2) with a spread narrower than the specification width. All processes have inherent statistical variability which can be evaluated by statistical methods.. These results are easily analyzed and displayed using SPC software. This is a complete & detailed course in Process Capability Analysis with an illustration of practical examples in Minitab. The within and overall curves on the histogram are normal distribution curves that are generated using the process mean and different estimates of process … Using the values previously noted, the vehicle capabilities are: Handpicked Content: Process Capability - The Basics: Part 1. … Verify that the process is stable 2. Normal Capability Analysis. Capability is determined by comparing the width of the process variation (VOP Voice of the Process.)

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