Message-ID: <loom.20150103T184152-823@post.gmane.org>
Date: 2015-01-03T17:46:36Z
From: Ben Bolker
Subject: EC50 Calculations using Insect Size
Katie Harding <katiemharding at ...> writes:
>
> I have insect size measurements that I fit to a Weibull curve
> to determine EC50. ?This is my model
> fit:model=nls(y~SSweibull(x,asym,drop,lrc,pwr))
> I need to use the untreated buffer samples as the maximum size
> (the left most asymptope). ?If I put the doses
> as 0,1,2,4,8,16,32,64,128 ppm,
> I get error messages indicating that 0 values for x are inappropriate.
> ?This is the error message:Error in qr.default(.swts *
> attr(rhs, "gradient")) :?? NA/NaN/Inf in
> foreign function call (arg 1)?
> I found that I get more consistent results when I put
> 0.01 for the dose of the untreated controls. ?I was
> wondering if there was a convention to follow when using
> untreated buffer samples in EC50 calculations. ?Cheers,Katie
Are you measuring a mortality response? If so, you should probably
be allowing the responses to be binomial rather than using least-squares
(as nls() does).
The drc package from CRAN might be helpful (it has a built-in
weibull response function).
Can you be slightly more specific/give a reproducible example?