I have a DOE layout with 3 factors with levels of 3,3 and 5. I am using Minitab 15(I am new to Minitab) to analyze the results. I cannot get Minitab to display a pareto chart or normal distribution chart. I have followed the Minitab instructions but but can not get it work properly. I think it might have something to do with the the factor levels not being the same. Any ideas?

Thanks.

I don't have access to Minitab at the moment, but there should be a GRAPHS button where you can check the graphs that you want. I think the Half Normal plot is the default, but you can change it under Options.

I played a round a little in Minitab and think I found the problem.

If you do a single trial at every setting AND analyze every interaction, then you run out of degrees of freedom. There are a two fairly simple solutions you can try:


get rid of the A*B*C interaction from the analysis (you can also get rid of of some of the 2-factor interactions if you have good reason to expect they are not going to be significant).
run some of the trials again - for example repeat the center point a few times and add the results as lines 46, 47, ...

Either of these should allow the analysis to proceed.


Hope that will help you get the answers you want from your DOE!

Tim F

Thanks Tim.

I took out the 3 factor interaction and the residual plots now look good, but I still can't find the pareto chart showing the significant factors/interactions. In my orginal design I put in 5 repeats of a center point (so the DOE had 50 trials) but still could not get a good plot. I am not sure if I understand how to determine the degrees of freedom and how to use that when laying out/analyzing the design.

Thanks again.