Q: If i try to log-transform both sides of the PML model using statement observe(log(CObs) = log(C)+CEps) I get a warning and it does not work. Answer: You cannot have a function on the left hand side of the “=”. In a built-in or graphical model you can just select a log-additive error model. In a text model you can write an error model such as the following. observe(CObs = C * exp(CEps)) or observe(CObs = exp(log(C) + CEps) ) Phoenix internally takes the log of both sides, and then sees if the derivative with respect to CEps is 1. If so, and if there is only one observe statement, then it does log-transformation.