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42 3 Treatment structureTables of contrastsVariate: yieldblock.*Units* stratumN contrasts0 versus 180 and 230 0.754, s.e. 0.0749, ss.div. 8.00S contrastsLin 0.0094, s.e. 0.00239, ss.div. 7875.Quad -0.00042, s.e. 0.000199, ss.div. 1131429.Deviations, e.s.e. 0.0706, ss.div. 9.00N.S contrastsS 0 10 20 40-0.028 0.074 -0.055 0.0090 versus 180 and 230.Lin 0.0173, s.e. 0.00506, ss.div. 1750.0 versus 180 and 230.Quad -0.00042, s.e. 0.000422, ss.div. 251429.The interaction between nitrogen and sulphur is now partitioned according to thenitrogen comparison. The line "0 versus 180 and 230.Lin" assesses the effect offitting two different linear relationships, one for each level 0 of nitrogen, and one forlevels 180 and 230 of nitrogen, instead of a single overall linear contrast. Similarly, theline "0 versus 180 and 230.Quad" represents the difference between the twoquadratic contrasts. So you can define contrasts on any treatment factor, and GenStat willautomatically estimate their interactions.As explained in Section 1.6, to fit polynomial contrasts, GenStat calculates orthogonalpolynomials and does a multiple regression of the effects of factor using the polynomialsas x-variates. Regression contrasts are similar to polynomial contrasts, except that hereyou can supply your own matrix of x-variates. GenStat orthogonalizes the x-variates foryou, so that each one represents the effect adding this x-variable to a model containingall the earlier ones.