<P> Analysis of variance and regression analysis are used to test for significant interactions . </P> <P> Is the yield of good cookies affected by the baking temperature and time in the oven? The table shows data for 8 batches of cookies . </P> <P> The data show that the yield of good cookies is best when either (i) temperature is high and time in the oven is short, or (ii) temperature is low and time in the oven is long . If the cookies are left in the oven for a long time at a high temperature, there are burnt cookies and the yield is low . </P> <P> From the graph and the data, it is clear that the lines are not parallel, indicating that there is an interaction . This can be tested using analysis of variance (ANOVA). The first ANOVA model will not include the interaction term . That is, the first ANOVA model ignores possible interaction . The second ANOVA model will include the interaction term . That is, the second ANOVA model explicitly performs a hypothesis test for interaction . </P>

When does an interaction occur in an experiment