Industrial Statistics with Minitab

21.3 Mattresses (2nd Part)

After removing the special causes – mainly due to problems with the temperature in the annealing furnace (at the first hour of the morning, the necessary value was not yet reached) and to a change of supplier – the data collection is continued the following week and the values of the variables named Length 2, Time 2, and Day 2 are obtained.

We want to know whether the problems have been solved, whether the process is in a state of control and whether it is capable.

With these data and all default options, draw again a - R chart:

The process is, basically, in a state of control. Nonetheless, the point detected outside the control limits needs to be investigated in order to find the cause that produced that behaviour. For a more detailed study, use the Capability Sixpack tool.

Stat > Quality Tools > Capability Sixpack > Normal

Clicking on Options, enter 30 in Number of subgroups display: Last so that the points of all samples are displayed in the graph named Last 30 subgroups, in such a way that they are aligned with their means and ranges.

The data follow a normal distribution reasonably well and the process is, basically, in state of control. ...