When a factor has only two levels in an analysis of variance, Genstat prints the difference between the two main effects. This difference is called the response. For interaction terms whose factors all have only two levels, there are two forms of response controlled by the TWOLEVEL option of ANOVA and ADISPLAY. With the default setting, response, Genstat calculates the response for an interaction between two factors as the difference between the two main-effect responses, and so on; this is the form described in most textbooks. Alternatively, the setting Yates provides the form defined by Yates (1937, The design and analysis of factorial experiments, Technical Communication No. 35 of the Commonwealth Bureau of Soils, Commonwealth Agricultural Bureaux, Farnham Royal), in which the differences are divided by 2m-1 where m is the number of factors in the term.