3 An experiment tested the effects of package design and shelf display on the likelihood of buying a breakfast cereal. Package design and shelf display were varied at two levels each, resulting in a 2 * 2 design. Purchase likelihood was measured on a seven-point scale. The results are partially described in the following table:

Source of variation Sum of squares df Mean square F Sig. of F v2 Package design 68.76 1 Shelf display 320.19 1 Two-way interaction 55.05 1 Residual error 176.00 40 a Complete the table by calculating the mean square, F, significance of F, and v2 values.

b How should the main effects be interpreted?