I still do not understand how to express a first order model. Can you please simplify this explanation that someone provided me. My understanding is that the first order model is the one that has not interaction.This would be the y=B1x1+b2x2+e example, where two variables with no interaction are used.The next orders would include the interaction, like y=B3x1x2+b4x1x3, where x1 and x2 interact, and x1 and x3 interact.Then you would combine them all together to show y=B1x1+b2x2+b3x1x2+b4x1x3+e as the whole formula.showing first and second order interaction.