QI Macros Lesson 4 - Control Charts and Histograms for Stability and Capability Analysis | |||||||||||||||||||
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A stable process produces PREDICTABLE RESULTS CONSISTENTLY. Process stability can be easily determined using control charts. A control chart is a line graph of your data (the same line graph used to identify and focus your problem) with average and sigma lines to determine stability. The average and sigma lines (+ 1, 2 and 3 sigma) are calculated from the data. The Upper Control Limit (UCL) is the +3 sigma line and the Lower Control Limit (LCL) is the -3 sigma line. 99.7% of all data points will fall between these two limits. The QI Macros for Excel identify unstable points or trends for you and highlight them in red. Rules for determining unstable conditions in QI Macros 2006 and earlier versions are based on the following rules from Montgomery 4th edition: QI Macros 2007 and later versions default to Montgomery's rules but also give you the flexiblilty to choose from other rules or define your own:
See our control chart rules article for instructions on how to change rules in the QI Macros.
One area that many people struggle with is choosing the right control chart. The right chart is based on the type of data, attribute (counted) or variable (measured), and the sample size. The QI Macros Control Chart Wizard can analyze your data and select the right control chart for you. Simply click and drag over your data to select it and then select Control Chart Wizard from the pull down menu. The wizard can select the right chart and only needs to ask one question when it is trying to determine between a p and u chart. If you want to understand how to choose the right chart, here are some tips: For attribute (i.e., integer or counted defects) data use np, p, c and u charts
For variable or measured data (i.e., decimal) such as time, cost, length, weight, etc. use XMR, XbarR, and XbarS Charts
These rules are summarized and shown pictorially in our SPC Quick Reference Card. Download it now. If you still need more help understanding how to choose the right chart see Lesson 8 or consider the following:
Processes are never perfect. So, how can you tell if a process is stable? Common and special causes of variation make the process perform differently in different situations. Common causes of variation happen all the time, every day. Getting from your home to school or work takes varying amounts of time because of traffic or transportation delays. Processes that are "out of control" need to be stabilized before they can be improved. Special causes, require immediate cause-effect analysis to eliminate the special cause of variation. A blizzard, a traffic accident, a chemical spill or other freak occurrence would be a special cause of variation.
The Exponentially Weighted Moving Average (EWMA) and Cusum charts detect process changes more rapidly than regular control charts. So you might choose one of these charts to monitor for small subtle process shifts (less than 1.5 sigma).
A capable process MEETS THE CUSTOMER'S REQUIREMENTS 100% OF THE TIME. The Upper and Lower SPECIFICATION limits (USL and LSL) are determined from the customer's requirements. The histogram is the correct tool to analyze process capability. The QI Macros histogram will calculate the process capability measures (Cp, Cpk, Pp and Ppk) based on your data and on the specification limits. To run a histogram, select your data and then select histogram from the QI Macros pull down menu. The macro will prompt you for the spec limits: If you don't know the specification limits, the QI Macros will calculate them for you. You can click ok to use the default, input your spec limit or click cancel if you have one-sided or uni-lateral spec limits. Determining Capability Using Attribute Data The capability of counted (i.e. attribute) data like defects, indivisible integers only, is zero defects. Customers hate defects, outages, etc. The capability of measured (i.e. variable) data like time, money, age, length, weight, etc. is determined using the customer's specifications and a histogram. Determining Capability Using Variable Data When a customer defines an upper and/or a lower specification limit for a product or service, whether it's the diameter of a shaft or the time in line at a fast food restaurant, all points within the two limits are considered "good."
Check out our Six Sigma Conversion Table For more information about running histograms in the QI Macros see Lesson 10. For more information about the formulas and how the capability indicies are calculated see our formulas page or Histogram Whitepaper. You can calculate Cp, Cpk and Pp, Ppk without running a histogram using the Cp Cpk template. Access the template under Fill in the Blanks Templates, SPC Charts.
The box-and-whisker and multivari chart can be used to show variation between different production lines, machines etc. You can run these charts using the QI Macros pull down menu. Test data is located at C/QIMacros/Testdata on your computer.
If you are spending too much time updating charts every month, you might consider our Fill-in-the-Blanks Dashboard System to automate your efforts or you can build one yourself using the Creating Dashboards and Scorecards ebook. If you want to better understand and use SPC to monitor your process performance consider our One Day SPC Workshop, 1 hour SPC Simplified Training Video or SPC Simplified Book. In Lesson #5 we will cover Other Advanced Features and Tips. View previous lesson .........View next lesson To sign up for this series tell your friends to send an email to qimacros@aweber.com
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It contains over 70 fill in the blank templates such as the Ishikawa diagram, QFD, DOE, FMEA, PPAP, and Gage R&R for MSA. Performs ANOVA, t-test, F-test, and regression analysis.
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