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## ←GenStat  ~  Genstat: Non-linear models

james brown (JIC)
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 Good afternoon Genstatters, I’m fitting logistic curves to dose-response data: Z = A+C/(1+EXP(-B(X-M))) Z = observed growth of parasite, A = lower asymptote of logistic curve (normally growth = 0), C = estimate of growth without pesticide, M = ED50 & B = slope parameter. I have data for a set of fungal strains. I can do most of what I want to do except that I want to fit curves with a common value of B but different values of M for different strains. The problem is that these are both non-linear parameters and Genstat seems to want to estimate either common values of B and M for all strains or separate values of both B and M for each strain. In the menus, Stats > Regression > Standard Curves gives me the option of setting Group = Strain. Step-by-step, this: Gives common estimates of A, C, B and M for all strains,Gives separate estimates of A and C with common estimates of B and M (first A estimated separately, then A and C both separately),Then estimates both non-linear parameters separately, which isn’t really what I want. The commands in the input window are: TERMS log10Dose*Strain FITCURVE [PRINT=model,summary,estimates,accumulated; CURVE=logistic; CONSTANT=estimate; FPROB=yes] log10Dose ADD [PRINT=model,summary,estimates,accumulated; FPROB=yes] Strain ADD [PRINT=model,summary,estimates,accumulated; FPROB=yes] Strain.log10Dose ADD [PRINT=model,summary,estimates,accumulated; NONLINEAR=separate; FPROB=yes] And the parameters estimates from the last two commands are: Estimates of parameters ----------------------- Parameter estimate s.e. B -6.144 0.287 M 0.86170 0.00968 C Strain V295L-1 1.545 A Strain V295L-1 0.09750 C Strain V295L-2 1.657 A Strain V295L-2 -0.04056 C Strain V295L-3 1.599 A Strain V295L-3 -0.008739 C Strain WT 1.634 A Strain WT -0.04959 Then Estimates of parameters ----------------------- Parameter estimate s.e. B Strain V295L-1 -5.205 0.587 M Strain V295L-1 0.9747 0.0361 C Strain V295L-1 1.829 0.135 A Strain V295L-1 -0.197 0.129 B Strain V295L-2 -7.488 0.761 M Strain V295L-2 0.8344 0.0162 C Strain V295L-2 1.5521 0.0545 A Strain V295L-2 0.0557 0.0495 B Strain V295L-3 -8.074 0.872 M Strain V295L-3 0.8454 0.0155 C Strain V295L-3 1.4915 0.0522 A Strain V295L-3 0.0820 0.0475 B Strain WT -4.692 0.476 M Strain WT 0.8160 0.0299 C Strain WT 1.6943 0.0906 A Strain WT -0.0597 0.0832 Can anyone suggest a way of tweaking the commands so I can get separate estimates of A, C and M but a common estimate of B? Thanks and best wishes, James Brown Prof James Brown Crop Genetics Dept, John Innes Centre Norwich NR4 7UH, England Post generated using Mail2Forum (http://www.mail2forum.com)

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