Hmmm.this could be why, in Minitab training, our instructors recommend using the Capability Sixpack first. In the case above, the subgroup size was 1 and MinitabвЂ™s Capability Sixpack in Stat > Quality Tools > Capability Sixpack > Normal will plot the data on a control chart for individual observations, an I-chart: So what is causing the within subgroup variation to be so small? Another graph in Minitab can give us the answer: The Capability Sixpack. The within subgroup variation of 0.0777 is very small relative to the overall standard deviation.

This is the StDev (Within) shown on the left side of the graph. Notice that the вЂwithin subgroupвЂ™ variation in the capability output is represented by the tall dashed line in the middle of the histogram. So what explains the odd output for the capability analysis? Yet if we use the exact same data to make a histogram using the Graph menu, we see that things donвЂ™t look so bad: WhatвЂ™s going on here? The Cpk seems unrealistic at 42.68, the 'within' fit line is tall and narrow, and the bars on the histogram are all smashed down. When you look at the results, the Cpk is huge and the histogram in the output looks strange: You generate the output using Minitab Statistical Software. IвЂ™m sharing the details in this post so that youвЂ™ll know where to look if you encounter a similar situation.

Here is a scenario involving process capability that weвЂ™ve seen from time to time in Minitab's technical support department.

Minitab histograms