Choose a design for the screening experiment

The choice of design will depend on how detailed the desired infonnation is to be. If it is strongly suspected that certain variables will interact, it is recommended that terms for these interaction should be included in the model, and a design be selected which can estimate these parameters. Use a Resolution V design by which each two-variable interaction can be estimated, or use a Resolution TV design which will confound the interaction effect. These can then be isolated by complementary runs. 

Full factorial designs Such designs are the best choice when the number of variables is four, or less. A full four-variable factorial design gives estimates of all main effects and two-variable interaction effects, and also an estimate of the experimental enor variance. This is obtained firom the residual sum of squares after a least squares fit of a second-order interaction model, see (Example Catalytic hydrogenation, p. 112). A full factoral design should be used if individual estimates of the interaction effects are desired. Otherwise, it is recommended first to run a half fraction 2 (I = 1234), and then run the complementary fraction, if necessary, (see Example Synthesis of a semicarbazide, p. 135). 

Fractional factorial designs The first choice when there are more than four variables, should always be to attempt a fractional factorial design. The confounding patterns are easily obtained firom the generators. It is also easy to append complementary runs to resolve any ambiguities. 

Resolution III designs can be constructed to accomodate three variables in four runs, up to seven variables in eight runs etc. Such designs can be augmented to Resolution IV designs by fold-over. Resolution IV designs are useful, since they make it possible to detect presence of strong interaction effects. 

of course, possible to construct Resolution IV designs directly, by using three-variable interaction column to define the extra variables. This is advantageous with eight variables, which by this technique can give a resolution IV design in 16 runs, 2 (I = 1235 = 1246 = 1347 = 2348). 

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