Process Control for control charts

Variable

For an Individuals Chart, I-MR Chart, and C Chart, all the measurements for a variable must be in one column. In Variable, enter one column of measurement data.

All observations for a chart are in one column and Observations for a subgroup are in one row of columns

Variable and Subgroup sizes

For a Laney P' Chart, U Chart, and P Chart, you must enter a variable column and specify the subgroup size. In Variable, enter the column that contains the process data.

Weight of EWMA

In Weight of EWMA, enter the weight to use in the exponentially weighted moving average. The value must be between 0 and 1. If you change the default weight (0.2) and the number of standard deviations for the control limits, you can construct a chart with specific properties. Choose the weight based on how small of a shift you want to detect and how fast you want to detect that shift while maintaining an acceptable false alarm rate. You can choose combinations of these two parameters by using an ARL (average run length) table. See Lucas et al.¹ for an extensive table.

Specify observations to estimate parameters

If some observations have erratic data because of special causes that you have corrected, you can omit these observations from the calculations to avoid incorrect parameter estimates.

Method for estimating standard deviation

Select Use unbiasing constant to eliminate bias that can occur when you estimate certain process parameters. Often, the choice to use unbiasing constants depends on company policy or industry standards.

Length of moving range

Enter the number of observations that are used to calculate the moving range. By default, a length of 2 is used because consecutive values have the greatest chance of being alike. The length must be 100.

If the data are cyclical, consider using a moving range of a different length. For example, if you collect quarterly data, consider using a moving range of length 4 to ensure that one observation from each season is included in the calculation.

Use Nelson estimate

On the Estimate tab of the I-MR Options subdialog, select Use Nelson estimate to correct for unusually large moving range values in the calculation of the control limits. The procedure is similar to the procedure proposed by Nelson. Connect eliminates any moving range values that are more than 3 standard deviations larger than the average moving range, then recalculates the average moving range and the control limits.

By default, Connect calculates the control limits using the actual subgroup sizes. When the subgroup sizes differ, the control limits are uneven, but you can force the control limits to be straight. Under When subgroup sizes are unequal, calculate control limits, select Assuming all subgroups have size, and enter a subgroup size.

For example, the data for the following charts is the same, but the control limits for the second chart were calculated based on a fixed subgroup size.

Actual subgroup sizes

Fixed subgroup size

On the Options subdialog, Connect provides one to eight tests for special causes, depending on the control chart. By default, Connect uses only Test 1. Select additional tests based on company or industry standards. Use the tests to determine which observations to investigate, and to identify the specific patterns and trends in your data.

In the drop-down list, specify whether to perform some, all, or no tests for special causes. You can make each test more or less sensitive by changing the value of K.

1 point > K standard deviations from center line

Test 1 identifies subgroups that are unusual compared to other subgroups. Test 1 is universally recognized as necessary for detecting out-of-control situations. If small shifts in the process are of interest, you can use Test 2 to supplement Test 1 in order to create a control chart that has greater sensitivity.

K points in a row on same side of center line

Test 2 identifies shifts in the process centering or variation. If small shifts in the process are of interest, you can use Test 2 to supplement Test 1 in order to create a control chart that has greater sensitivity.

K points in a row, all increasing or all decreasing

Test 3 detects trends. This test looks for a long series of consecutive points that consistently increase in value or decrease in value.

K points in a row, alternating up and down

Test 4 detects systematic variation. You want the pattern of variation in a process to be random, but a point that fails Test 4 might indicate that the pattern of variation is predictable.

K out of K+1 points > 2 standard deviations from center line (same side)

Test 5 detects small shifts in the process.

K out of K+1 points > 1 standard deviation from center line (same side)

Test 6 detects small shifts in the process.

K points in a row within 1 standard deviation of center line (either side)

Test 7 detects a pattern of variation that is sometimes mistaken as evidence of good control. This test detects control limits that are too wide. Control limits that are too wide are often caused by stratified data, which occur when a systematic source of variation is present within each subgroup.

K points in a row > 1 standard deviation from center line (either side)

Test 8 detects a mixture pattern. In a mixture pattern, the points tend to fall away from the center line and instead fall near the control limits.

Reference lines

To add a reference line, after you create control chart, select the Edit icon . On the right navigation, select the Graph Options tab and select Add for the control chart you want. You can specify the following options for the line.

Name

The name you specify appears on the right side of the chart beside the reference line.

Value

Enter a value on the y-axis for the reference line.

Operator

Set an alert to the control chart. You can have the alert notify you if there is a point greater than or less than the reference line. Or you can turn alarms off for the reference line.

Note

If you turn on an alert for a reference line, you still need to create a new alert to receive a notification. See the following section for more information on how to set up a new alert.

Line Color

Specify a color for the reference line.

After you add a reference line, you can use the Graph Options tab to change the value, color, and name of the line, change the alert status, or delete the line.

Alerts

To add an alert after you a create control chart, select the New Alert icon . You only receive alerts for points you affect after you add new data or modify existing data.

Name

The name of the alert.

Type

The type of alert. You can choose from an email, text message, or Connect notification.

To

Select one or more persons to receive the alert.

For emails and texts, use the dropdown list to select a user to add them to the alert. If you want to add an email or phone number for a person that is not in the dropdown list, type it into the box. After Connect verifies that it is valid, select Press Enter to Add Email or Press Enter to Add Phone Number to add it to the alert.

United States phone numbers must be 10 digits with any combination of parentheses, periods, dashes, and spaces as delimiters. Parentheses can only be used on the area code. International phone numbers must start with a + symbol and can contain up to 15 digits with no spaces or delimiters.

Subject Line

Enter the subject line of the email when you select Email as the type of alert.

Message

Create the message the alert sends.

Tests for special causes

Set the alert to the tests that you select. You can only select tests that you selected on the Control Chart options sub dialog when you created the chart. You can edit the tests for special causes for existing control charts by selecting the Open icon , selecting the Options button, then selecting the Tests tab.

Reference Lines

Set the alert to any references lines you added to the control chart. You can use the Graph Options tab on the right navigation to change the value and the alert status of the line.

To enable, disable, edit, or delete an alert after you create it, select the ellipsis in the header bar and select Manage Alerts.