Time Between Charts

Jay Arthur
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KnowWare International, Inc.
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When defect rates become low (closer to 3.4 parts per million), you can use the QI Macros to draw a "time between" chart. Since the time between defects (or units between failures) approximates an exponential distribution, you can transform the data using BoxCox (lambda = 1/3.6 =0.2777) and graph the result.

BoxCox

Here's some data from Montgomery's Intro to SPC that can be transformed using boxcox to a normal distribution using a factor of 1/3.6 = 0.2777 (Weibull factor):

To do this in the QI Macros:

1. Select the time between or number of units between failures data (B1:B21).

2. Click on the QI Macros pulldown menu and select Data Transformation-BoxCox:

3. Enter the transformation factor (0.2777):

Time Between XmR Chart

Then select the transformed data and run an XmR Chart:

Time Between Template

You can also use the QI Macros t Chart template, which converts the transformed time back into regular time before plotting the chart.:

To purchase the QI Macros

If you already own the QI Macros, you can purchase an upgrade. You must purchase one upgrade for each license you own.

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