fit_model_triphasic: Fit model and obtain parameter estimates
In ushr: Understanding Suppression of HIV

This function fits the triphasic model to the processed data and extracts the best-fit parameters.

1
2
3

fit_model_triphasic(data, id_vector, param_names, initial_params,
  free_param_index, n_min_triphasic, forward_param_transform_fn,
  inv_param_transform_fn, searchmethod)

`data`	dataframe with columns for each subject's identifier ('id'), viral load measurements ('vl'), and timing of sampling ('time')
`id_vector`	vector of identifiers corresponding to the subjects to be fitted.
`param_names`	names of parameter vector.
`initial_params`	named vector of the initial parameter guess.
`free_param_index`	logical vector indicating whether the parameters A, delta, A_b, delta_b, B, gamma are to be recovered. This should be c(TRUE, TRUE, TRUE, TRUE, TRUE, TRUE) for the triphasic model.
`n_min_triphasic`	the minimum number of data points required to fit the triphasic model.
`forward_param_transform_fn`	list of transformation functions to be used when fitting the model in optim. Defaults to log transformations for all parameters (to allow unconstrained optimization).
`inv_param_transform_fn`	list of transformation functions to be used when back-transforming the transformed parameters. Should be the inverse of the forward transformation functions. Defaults to exponential.
`searchmethod`	optimization algorithm to be used in optim. Defaults to Nelder-Mead.