The shortage of parameters to estimate all six parameters creates potential problems with modeling G × E using the cross-product term as delineated in Eq. (1) when there are three G levels, as it imposes a number of constraints, which are generally unacknowledged and may not be accurate for the data. The first is that the lines are always ordered; the second is that the slope differences between adjacent regression lines are always the same; and the third is that the lines always cross at the same point or are parallel. Note that the correspondence between the δij and βi coefficients as derived above means that the equations for G = 0, 1, 2 are, respectively: P∣(G=0)=β0+β2E,P∣(G=1)=(β0+β1)+(β2+β3)E,P∣(G=2)=(β0+2β1)+(β2+2β3)E.
