equal to 0 is then f(x, (b, d, e)) = d exp{− exp{b(log(x)− e)}}. Besides the Gompertz model and the logistic model, Brain-Cousens’ model (Brain and...

equal to 0 is then f(x, (b, d, e)) = d exp{− exp{b(log(x)− e)}}. Besides the Gompertz model and the logistic model, Brain-Cousens’ model (Brain and Cousens 1989) is used in situations where hormesis is present: For small doses the herbicide has the adverse effect resulting in response values above the level of the control group (Calabrese and Baldwin 2001, 2003). Brain-Cousens model is obtained by modifying the four-parameter logistic model f(x, (b, c, d, e)) = c + d + fx− c 1 + exp {b( log(x)− log(e))} (4) adding the linear term fx in the numerator. Again we could also consider the modification where the lower limit is set equal to 0. Notice that the b and e parameters are different in (1), (2), (3) and (4), respectively. In this paper we assume that the response is a decreasing function of the dose (from maximum response to lower limit), corresponding to positive b, but the models also apply to situation where the response is increasing with the dose.