When points fail Minitab's tests, they are marked in red. In this MR chart, none of the individual points fall outside the lower and upper control limits of 0 and 0.4983, respectively. But when the MR chart is in control, an out-of-control I chart does indicate changes in the process center. In that case, any unusual points on the I chart may result from unstable variation, not changes in the process center. The reason you want to check this first is that if the MR chart is out of control, the I-chart control limits won't be accurate. This chart reveals if the process variation is in or out of control. It might seem counterintuitive, but you should examine the MR chart at the bottom first. In the dropdown menu, select "Perform all tests for special causes," and then OK out of the dialog.Īfter you press OK, Minitab generates your I-MR chart: These tests highlight points that exceed the control limits and detect specific patterns in the data. Let's assume that we want to detect any possible special-cause variation. (If you have more than one variable you want to chart, you can enter more than one column here and Minitab will produce multiple I-MR charts simultaneously.) If you want to add labels, divide the data into stages, and more, you can do that in the "I-MR Options" subdialog. Now select Stat > Control Charts > Variables Charts for Individuals > I-MR from the menu, and choose pH as the Variable. If you don't already have Minitab Statistical Software, you can use the free trial.) (You can download this data set here to follow along. To prepare that data for the I-MR chart, just enter those measurements, in order, in a single column of a Minitab worksheet. ![]() You record pH measurements for 25 consecutive batches. Since you are collecting individual samples rather than subgroups, the I-MR chart can help. You record a single pH measurement per batch. Suppose the chemical company you work for makes a custom solution, and you need to assess whether its pH value is consistent over time. The green line represents the process mean and process variation on the I and MR portions of the chart, respectively, while the red lines represent the upper and lower control limits. It calculates the variation of your process using ranges of two (or more) successive observations, and plots them. The graph at the bottom is called a Moving Range (MR) chart. It shows you the value of each observation (the individuals), and helps you assess the center of the process. The graph's top part is an Individuals (I) chart. The I-MR is actually the combination of two different charts in a single presentation. Now that we know what the I-MR chart can do, let's consider what it is. If your improvements had a big impact, how do you show your stakeholders and higher-ups? Before-and-after I-MR charts provide powerful visual proof.
0 Comments
Leave a Reply. |