Semifolding 2k-p designs

This article addresses the varied possibilities for following a two-level fractional factorial with another fractional factorial half the size of the original experiment. Although follow-up fractions of the same size as an original experiment are common practice, in many situations a smaller followup experiment will suffice. Peter John coined the term "semifolding" to describe using half of a foldover design. Existing literature does include brief mention and examples of semifolding but no thorough development of this follow-up strategy. After a quick examination of the estimation details for semifolding the 2(IV)(4-1) design, we focus on following 16-run fractions with a semifold design of eight runs. Two such examples are considered-one in which the initial fraction is resolution IV, the other resolution III. A general result is proven for semifolding 2(IV)(k-p) designs. Conducting full foldover designs in two blocks is also recommended.