Main effects are the impact of changing a single variable in a multivariate test. When a test only monitors main effects the assumption is made that there is no interaction between individual variables (see Variable interaction).
If this is not the case there is a danger that the winning recipe will not deliver the expected uplift because of the interaction between individual variables. This is a drawback of fractional factorial multivariate tests which assume there is no relationship between the independent variables.
Comparison of Full Factorial and Fractional Factorial DOE
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