CRS.4a {drc} | R Documentation |
'CRS.4a', 'CRS.4b' and 'CRS.4c' provide the Cedergreen-Ritz-Streibig modified log-logistic model for describing hormesis with the lower limit equal to 0.
'UCRS.4a', 'UCRS.4b' and 'UCRS.4c' provide the Cedergreen-Ritz-Streibig modified log-logistic model for describing u-shaped hormesis with the lower limit equal to 0.
CRS.4a(names = c("b", "d", "e", "f")) UCRS.4a(names = c("b", "d", "e", "f"))
names |
a vector of character strings giving the names of the parameters. The default is reasonable (see above). |
The model is given by the expression
f(x) = 0 + frac{d-0+f exp(-1/x)}{1+exp(b(log(x)-log(e)))}
which is a five-parameter model.
It is a modification of the four-parameter logistic curve to take hormesis into account.
The u-shaped model is given by the expression
f(x) = 0 + d - frac{d-0+f exp(-1/x^{α})}{1+exp(b(log(x)-log(e)))}
The a,b,c models are obtained by setting alpha equal to 1, 0.5 and 0.25, respectively.
See cedergreen
.
This function is for use with the function drm
.
Christian Ritz
See the reference under cedergreen
.
Similar functions are CRS.5a
and UCRS.5a
,
but with an extra parameter for the lower limit.
## Fitting modified logistic models lettuce.crsm1 <- drm(lettuce[,c(2,1)], fct=CRS.4a()) summary(lettuce.crsm1) ED(lettuce.crsm1, c(50)) lettuce.crsm2 <- drm(lettuce[,c(2,1)], fct=CRS.4b()) summary(lettuce.crsm2) ED(lettuce.crsm2, c(50)) lettuce.crsm3 <- drm(lettuce[,c(2,1)], fct=CRS.4c()) summary(lettuce.crsm3) ED(lettuce.crsm3, c(50))