Miner, thank you very much.

Yes I used MINITAB, and ran standard order 2^k-p fractional factorial with 3 center point. I have 4 factor and 3 center point.

The issue here, what are the rules and regulation that my first order can proceed to second order model miner?

Is that, I need to consider the main effect or interaction or curvature for getting significant before go to second order analysis?

Sorry Miner, I'm now in completing master. I don't have any statistical background. I think, this forum help me very much..

Thanks for your reply.

Best regards,

Ridzwan Rahim