help with screening results

Question

Hi All,

Ive carried out a screening expriement looking into the effect of 6 factors on our response. I choose ro look at all two factor interactions when choosing my design. My problem is that there are a couple of significant two-factor interactions, but neither, or just one of the factors are significant as a main effect. Any ideas on what may be going on? Could the interactions be an alias for anything else?

Thanks!

bs1cad · Answer

Thanks for all the info. I think im slowly getting there! I plan to augment the design now to add center points to see any lack of fit and go from there

louv · Answer

Were there any center point runs included to see if there was a lack of fit and evidence of a potential polynomial term? Any replicates run?

unknown · Answer

It is very possible to have significant two-factor interactions but insignificant main effects.  Assuming you have a 2^k design, a main effect is the average of all values where factor i is equal to 1 minus the average of all values where factor i is equal to -1 (more or less).  If the effect of factor A depends of the level of factor B so that an interaction plot looks like an X then the main effect would be 0 but the interaction would be very strong.

However, given the aliasing structure of a resolution IV design the two factor interactions that appear significant may not be the actual interaction responsible for the cause effect.  I would take into account the aliasing structure and what factors appear to be significant to design a follow-up experiement that is resolution V+ to identify the most important effects.