Open the file Nonnormal Cycle Time2.xlsx. This contains continuous nonnormal data of process cycle times. We performed a Process Capability study with this data earlier in the Measure Phase, Part H.
Initially, we will ignore the nonnormality in the data and construct an Individuals Chart. Click SigmaXL > Control Charts > Individuals. Ensure that entire data table is selected. If not, check Use Entire Data Table. Click Next.
Select
Cycle Time (Minutes), click
Numeric Data Variable (Y) >>.
Select
Calculate Limits.
Check
Tests for Special Causes.
This chart clearly shows that the process is “out-of-control”. But is it really? Nonnormality can cause serious errors in the calculation of Individuals Chart control limits, triggering false alarms (Type I errors) or misses (Type II errors).
We will now construct Individuals Control Charts for nonnormal data. Select Sheet 1 Tab (or press F4). Click SigmaXL > Control Charts > Nonnormal > Individuals Nonnormal. Ensure that the entire data table is selected. If not, check Use Entire Data Table. Click Next.
Select
Cycle Time (Minutes), click
Numeric Data Variable (Y) >>.
We will use the default selection for
Transformation/Distribution Options: Box-Cox Transformation
with
Rounded Lambda.
Check
Tests for Special Causes
as shown:
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