To fold the design
main topic     see also 

1    In the Create Factorial Design dialog box, click Options.

2    Do one of the following, then click OK.

·    Choose Fold on all factors to make all main effects free from each other and all two-factor interactions.

·    Choose Fold just on factor and then choose a factor from the list to make the specified factor and all its two-factor interactions free from other main effects and two-factor interactions.