| logoff {VGAM} | R Documentation |
Computes the log transformation with an offset, including its inverse and the first two derivatives.
logoff(theta, earg = list(offset=0), inverse = FALSE, deriv = 0,
short = TRUE, tag = FALSE)
theta |
Numeric or character.
See below for further details.
|
earg |
List. Extra argument for passing in additional information.
The offset component of the list earg
is the offset value.
See Links for general information about earg.
|
inverse |
Logical. If TRUE the inverse function is computed. |
deriv |
Order of the derivative. Integer with value 0, 1 or 2. |
short |
Used for labelling the blurb slot of a
vglmff-class object. |
tag |
Used for labelling the linear/additive predictor in the
initialize slot of a vglmff-class object.
Contains a little more information if TRUE. |
The log-offset link function is very commonly used for parameters that
are greater than a certain value.
In particular, it is defined by log(theta+offset) where
offset is the offset value. For example,
if offset=0.5 then the value of theta is restricted
to be greater than -0.5.
Numerical values of theta close to -offset or out of range
result in
Inf, -Inf, NA or NaN.
The arguments short and tag are used only if
theta is character.
For deriv = 0, the log of theta+offset, i.e.,
log(theta+offset) when inverse = FALSE,
and if inverse = TRUE then
exp(theta)-offset.
For deriv = 1, then the function returns
d theta / d eta as a function of theta
if inverse = FALSE,
else if inverse = TRUE then it returns the reciprocal.
Here, all logarithms are natural logarithms, i.e., to base e.
The default means this function is identical to loge.
Numerical instability may occur when theta is
close to -offset.
Thomas W. Yee
McCullagh, P. and Nelder, J. A. (1989) Generalized Linear Models, 2nd ed. London: Chapman & Hall.
## Not run: logoff(seq(-0.2, 0.5, by=0.1)) logoff(seq(-0.2, 0.5, by=0.1), earg=list(offset=0.5)) log(seq(-0.2, 0.5, by=0.1) + 0.5) ## End(Not run)