The terms defining contrasts and generalized interactions are used in the theory of fractional replication of 2k designs. Explain what they mean. In an exploratory experiment, six factors, A, B, C, D, E, and F, are to be studied at two levels. However, only eight experimental units are available. Information is required on all main effects, and the factors are written in order of importance. Write down a list of eight treatments that form the fractional replicate of the 26 design. List the quantities that can be estimated if this design is used. What further assumption should be imposed to estimate the main effects?