lognormal: Log-normal distribution of times to an event

In drcte: Statistical Approaches for Time-to-Event Data in Agriculture

Log-normal distribution of times to an event

Description

This function provides the truncated log-normal cumulative distribution of event times, to be used to fit time-to-event models.

Usage

lognormal(fixed = c(NA, NA, NA), names = c("b", "d", "e"))
lognormalSurv(fixed = c(NA, NA, NA), names = c("b", "d", "e"))

Arguments

fixed	numeric vector. Specifies which parameters are fixed and at what value they are fixed. NAs for parameter that are not fixed. It should only be used to fix the 'd' parameter to 1, which provides the usual log-logistic distribution, with no fraction of individuals that are never expected to get the event .
names	a vector of character strings giving the names of the parameters. The default is reasonable.

Details

The log-normal distribution of germination times is parameterised as:

P(t) = d \cdot pnorm(b \cdot (log(t + 0.000001) - log(e)))

where 't' is the time and 'P(t)' is the proportion of individual with the event at time = t. The parameter 'e' is the median event time, 'd' is the fraction of individuals that are susceptible to get the event and 'b' is the inverse of the standard deviation on a log-scale. This parameterisation is the same as that found in the 'lnormal()' or 'LN.3()' functions in 'drc' and parameters are not given any link, which is the best option for clarity. In order to improve the estimation process and avoid errors, parameters can be given an appropriate link function. In 'lognormalSurv', 'e' is the median event time with a log-link, 'd' is the fraction of individuals that are susceptible to get the event (on a logit link) and 'b' is the standard deviation on a log-scale and a log-link. When d = 1, this parameterisation gives same results as the function 'survereg' in the 'survival package', with 'dist = "lognormal".

Value

The value returned is a list containing the nonlinear function, the self starter function and the parameter names.

Author(s)

Andrea Onofri

References

Kleinbaum, D.G., Klein, M., 2005. Survival analysis. Springer Science, USA.

Examples

library(drcte)
data(chickweed)
modb <- drmte(count ~ start + end,
               data = chickweed,
               fct = lognormal())

drcte documentation built on Aug. 22, 2025, 1:10 a.m.