 By convention each equation has one effect variable on the left hand side, and one or more cause variables on the right hand side. We also exclude from each equation any variable that makes no difference to the value of the effect variable. For example, the equation for Gas level could be written as Gas level = (Gas connected \(\times\) Gas knob) \(+\) (0 \(\times\) Meat cooked); but since the value of Meat cooked makes no difference to the value of Gas level in this equation, we omit the variable Meat cooked. A SEM is acyclic if the variables can be ordered so that variables never appear on the left hand side of an equation after they have appeared on the right. Our example is acyclic, as shown by the ordering of variables given above. In what follows, we will assume that SEMs are acyclic, unless stated otherwise.