Easy Creation of Control ChartsĬontrol charts have been used in statistical process control for decades, and are among the most commonly accessed tools available in statistical software packages. However, the car’s mean mileage, which is shown in the Individual Value chart displayed at the top of the graph, has seen a statistically significant change, moving from 19.12 MPG to just under 21 MPG. It shows that while the upper and lower control limits have shifted, the difference in variation before and after the change is not statistically significant. T he Moving Range chart, shown in the lower portion of the graph, illustrates the moving range of the data. The Assistant's Summary Report gives you the bottom-line results of the analysis. In this case, there are no concerns with the process mean and variation.
MINITAB EXPRESS CONTROL CHARTS HOW TO
Interpreting the Results of Your Data AnalysisĪfter you press OK, the Assistant produces a Diagnostic Report with detailed information about the analysis, as well as a Report Card, which provides guidance on how to interpret the results and flags potential problems. Complete the Before/After I-MR Chart dialog box as shown below: If you're following along with Minitab, the Maint column in the worksheet notes which MPG measurements were taken before and after DeWaggen started paying attention to maintenance.
The easiest way to handle this is to choose Assistant > Before/After Control Charts… to create a chart that makes it easy to see how the change affected both the mean and variance in the process. Since the data were collected both before and after this change, we want to account for it in the analysis. The owner bought the car three years ago, but he’s only done the recommended maintenance in the last year.
To create the most meaningful chart, we need to recall that a major change in how the vehicle is handled took place during the time the data were collected. Creating a Control Chart that Accounts for Process Changes How does his mileage measure up? And has his attention to maintenance in the past 12 months affected his car’s fuel economy? Let’s find out with the Assistant in Minitab Statistical Software. This year, though, he’s diligently performed recommended maintenance. However, the owner didn’t do any vehicle maintenance for the first two years he owned the car. Here's a simulated data set that collects 3 years’ worth of gas mileage records for a car that should get an average of 20 miles per gallon, according to the manufacturer’s estimates. This data can also reveal unusual variation that might indicate a problem you need to fix. Looking at your vehicle’s MPG data lets you see if your mileage is holding steady, declining, or rising over time. But knowing if you're getting good mileage requires some data analysis, which gives us a good opportunity to apply one of the common tools used in Six Sigma - the I-MR (individuals and moving range) control chart to daily life. Maximizing the miles you get per gallon of fuel saves money and helps the environment, too. Keeping your vehicle fueled up is expensive.