Degrees of freedom for t-value in DOE confidence interval calculations
Hello. I am new here. I have a 2^3 full factorial screening DOE with 3 centre-points. I am fitting a second-degree interactions model, so there are 4 df left for the Total Error; 2 for lack of fit, 2 for pure error. When I display the Parameter Estimates, the Std Error using the "Default" Error Specification is correct and the 95% CIs use a t-value of 2.776 based on 4 df. When I switch to "Pure Er...