---
title: Short demo of the multistart method
author: Johannes Ranke
date: Last change 19 September 2022 (rebuilt `r Sys.Date()`)
output:
html_document
vignette: >
%\VignetteEngine{knitr::rmarkdown}
%\VignetteIndexEntry{Short demo of the multistart method}
%\VignetteEncoding{UTF-8}
---
This is a vignette, because the multistart method does not seem to work in
pkgdown example code and I wanted to show the plots in the online docs.
The dimethenamid data from 2018 from seven soils is used as example data.
```{r}
library(mkin)
dmta_ds <- lapply(1:7, function(i) {
ds_i <- dimethenamid_2018$ds[[i]]$data
ds_i[ds_i$name == "DMTAP", "name"] <- "DMTA"
ds_i$time <- ds_i$time * dimethenamid_2018$f_time_norm[i]
ds_i
})
names(dmta_ds) <- sapply(dimethenamid_2018$ds, function(ds) ds$title)
dmta_ds[["Elliot"]] <- rbind(dmta_ds[["Elliot 1"]], dmta_ds[["Elliot 2"]])
dmta_ds[["Elliot 1"]] <- dmta_ds[["Elliot 2"]] <- NULL
```
First, we check the DFOP model with the two-component error model and
random effects for all degradation parameters.
```{r}
f_mmkin <- mmkin("DFOP", dmta_ds, error_model = "tc", cores = 7, quiet = TRUE)
f_saem_full <- saem(f_mmkin)
f_saem_full_multi <- multistart(f_saem_full, n = 16, cores = 16)
parhist(f_saem_full_multi)
```
We see that not all variability parameters are identifiable, most problematic
is the variance of k2. So we reduce the parameter distribution model by
removing the intersoil variability for this parameter.
```{r}
f_saem_reduced <- update(f_saem_full, covariance.model = diag(c(1, 1, 0, 1)))
f_saem_reduced_multi <- multistart(f_saem_reduced, n = 16, cores = 16)
parhist(f_saem_reduced_multi, lpos = "topright")
```
We can also analyse the log-likelihoods obtained in the multiple runs:
```{r}
llhist(f_saem_reduced_multi)
```
The one run with the lower likelihood is probably responsible for the outliers
in the boxplots above, and caused by some weird starting value combination. In
any case, the converged values from the original fit (red circles) appear
perfectly acceptable, supporting the choice of starting values made by mkin.
