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u Chart Help

A u control chart is used to monitor the variation in the number of defects. A defect occurs when something does not meet a preset specification. A u control chart is used with counting type attributes data (e.g., 0, 1, 2, 3).These are whole numbers. You can not have 1/2 defect. In addition, to use a u control chart, two other conditions must be true:

With a u control chart, we are often looking at an area of opportunity for defects to occur. A u control is similar to a c control chart, except that the area of opportunity for defects to occur is not constant. For more information on u control charts, please see our SPC Knowledge Base.

This page shows you how to make the u chart.The data can be downloaded at this link. This page contains the following:

Data Entry

You have a new device that is being used in a hospital. It rarely produces one kind of infection, but you want to track how often that happens. You measure the number of days that the device is in use each month and the number of infections that occurred. The data are shown below. You decide to make your inspection unit 1000 days. So, the u control chart will be monitoring the number of infections per 1000 device-days. Using the data below, the program will automatically determine the subgroup size (n) and the values of u. For Jan, n = number of device-days/1000 = 2500/1000 = 2.5 and u = c/n = 5/2.5 = 2.0

Enter the data into a spreadsheet as shown below. The data does not have to start in cell A1. It can be anywhere on the worksheet. The data can be in columns or rows. The program calculates the value of u.

Creating a New u Chart

This is all that is needed to make the chart. If you select OK at this point, the software will generate the control chart using the default options. Simple and quick.

The options on the input screen are:

Updating the u Chart with New Data

The u chart can be easily updated with new data after it has been added to the spreadsheet. Please see Updating Control Charts with New Data to see how the software finds the new data and updates the chart.

Changing the Options for the u Chart

You can change the current options for a u chart (e.g., changing the out of control tests) by selecting “Options” on the Updating/Options panel on the SPC for Excel Ribbon. See Changing Chart Options for more information

In addition, once a control chart is made, there are numerous actions you can take on the chart including splitting control limits, removing points from the calculations, adding comments, selecting the range on which to base control limits, etc. Please see Control Chart Actions for details.

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