Hello again, sorry for the notation. Again, I'm just a biologist!!! ;-)
But I'm enjoying this problem quite a bit! I'm very grateful for all the input. This is great. On 2003-07-25 08:38:00 -0400 Prof Brian Ripley <[EMAIL PROTECTED]> wrote: Answers: > Ymax is the maximum observation in your example, and also the observation at > zero. I was asking which you meant: if you meant Y at 0 (and I think you do) > then it is somewhat misleading notation. I will clean up my notation! > > You have a set of Poisson random variables Y_x at different values of x. > Poisson random variables have a mean (I am using standard statistical > terminilogy), so let's call that mu(x). Then you seem to want the value of x > such that mu(x) = mu(0)/2 *or* mu(x) = Y_0/2, OK, I want x for mu(x) = mu(0)/2. > that in your model mu(0) would be infinity, and so the > model cannot fit your data (finite values of Y_0 have zero probability). Correct, This is part of the problem! The model does not "hold" for X = 0. > the largest response because the "dose" is always detrimental to growth) > > The last is not true, given your assumptions, It could have the largest mean > response, but 0 is a possible value for Y_0. Yes, you are right, but then there is no growth, nad no LD50 value, so we reject this sample... > > Fit a model for the mean response (one that actually can fit your data), and > solve the estimated mu(x) = mu()/2 or Y_0/2. That gives you an estimate, and > the delta method will give your standard errors. Then you suggest using another model that will account for zero dose, OK. I think I saw something similar in another reply. I need to read it more carefully. -- Vincent Philion, M.Sc. agr. Phytopathologiste Institut de Recherche et de D�veloppement en Agroenvironnement (IRDA) 3300 Sicotte, St-Hyacinthe Qu�bec J2S 7B8 t�l�phone: 450-778-6522 poste 233 courriel: [EMAIL PROTECTED] Site internet : www.irda.qc.ca ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help
