param.init {SemiMarkov}

Defines the initial values of parameters for a semi-Markov model

Description

Function defining initial values of parameters of the waiting time distributions, probabilities of the Markov chain and optional regression coefficients associated with covariates. The function can either provides the default initial values (the same as those considered in the function semiMarkov) or can be used to specify particular initial values.

Usage

param.init(data = NULL, cov = NULL, states, mtrans, 
           cov_tra = NULL, cens = NULL, dist_init = NULL, 
           proba_init=NULL, coef_init = NULL)

Arguments

data

data frame of the form data.frame(id,state.h,state.j,time), where

• id: the individual identification number

• state.h: state left by the process

• state.j: state entered by the process

• time: waiting time in state state.h

The data.frame containts one row per transition (possibly several rows per patient). The data frame data is not needed if proba_init is provided.

cov

Optional data frame containing the covariates values.

states

A numeric vector giving the names of the states (names are values used in state.h).

mtrans

A quadratic matrix of characters describing the possible transitions and the distributions of waiting time. The rows represent the left states, and the columns represent the entered states. If an instantaneous transition is not allowed from state h to state j, then mtrans should have (h,j) entry FALSE, otherwise it should be "E" (or "Exp" or "Exponential") for Exponential distribution, "W" (or "Weibull") for Weibull distribution or "EW" (or "EWeibull" or "Exponentiated Weibull") for Exponentiated Weibull distribution. If TRUE is used instead of the name of the distribution, then a Weibull distribution is considered. By definition of a semi-Markov model, the transitions into the same state are not possible. The diagonal elements of mtrans must be set to FALSE otherwise the function will stop.

cov_tra

Optional list of vectors: a vector is associated with covariates included in the model. For a given covariate, the vector contains the transitions "hj" for which the covariate have an effect (only the transitions not equal to FALSE in mtrans are allowed). The effect of covariates can then be considered only on specific transitions. By default, the effects of covariates on all the possible transitions are included in a model.

cens

A character giving the code for censored observations in the column state.j of the data. Default is NULL which means that the censoring is defined as a transition fron state h to state h.

dist_init

Optional numeric vector giving the initial values of the distribution parameters. Default is 1 for each distribution parameter. The length of the vector depends on the chosen distribution, number of transitions and states.

proba_init

Optional numeric vector giving the initial values of the transition probabilities. The sum of the probabilities in the same raw must be equal to 1. According to semi-Markov model, the probability to stay in the same state must be equal to 0. The default values for the transition probabilities are estimated from the data. If data = NULL, the argument proba_init is obligatory.

coef_init

Optional numeric vector giving the initial values of the regression coefficients associated with the covariates. Default is 0 for each regression coefficient meaning no effect of the covariate.

Details

This function returns a data frame containing the initial values of parameters of a semi-Markov model. The model parameters are the distribution parameters, the transition probabilities of the Markov chain and the regression coefficients associated with covariates. The number of parameters depends on the chosen model: the distributions of the sojourn times, the number of states and the transitions between states specified with the matrix mtrans, the number of covariates (cov) and their effects or not on transitions (cov_tra).

The default initial values for the distribution parameters are fixed to 1. As the three possible distributions are nested for respective parameters equal to 1 (See details of the semiMarkov function), the initial distribution corresponds to the exponential distribution with parameter equal to 1 (whatever the chosen distribution). The default initial values for the regression coefficients are fixed to 0 meaning that the covariates have no effect on the hazard rates of the sojourn times. These initial values may be changed using the arguments dist_init and coef_init.

By default, the initial probabilities are calculated by simple proportions. The probability associated to the transition from h to j is estimed by the number of observed transitions from state h to state j divided by the total number of transitions from state h observed in the data. The results are displayed in matrix matrix.P. The number of parameters for transition probabilities is smaller than the number of possible transitions as the probabilities in the same row sum up to one. Considering this point and that the probability to stay in the same state is zero, the user can change the initial values using the argument proba_init.

Value

This function returns an object of class param.init to be used in functions semiMarkov and hazard. An object of class param.init consists of

nstates

The length of vector states interpreted as the number of possible states for the process.

table.state

A table, with starting states as rows and arrival states as columns, which provides the number of times that a transition between two states is observed. This argument is only returned when data is provided. It can be used to quickly summarize multi-state data.

Ncens

Number of individuals subjected to censoring.

matrix.P

Quadratic matrix, with starting states as rows and arrival states as columns, giving the default initial values for the transition propabilities of the embedded Markov chain. All diagonal values are zero. The sum of all probabilities of the same row is equal to one.

last

The largest duration observed in data if data is given.

Transition_matrix

A matrix containing the informations on the model definition : the possible transitions o and the distribution of waiting times for each transition (Exponential, Weibull or Exponentiated Weibull).

dist.init

A data frame giving the names of the parameters, transitions associated with and initial values of the distribution parameters.

proba.init

A data frame giving names of the parameters, transitions associated with and initial values of the probabilities of the embedded Markov chain.

coef.init

A data frame giving the names of covariates, transitions associated with and initial values of the regression coefficients.

Author(s)

Agnieszka Listwon-Krol

See Also

hazard, semiMarkov

Examples

## Asthma control data
data(asthma)

## Definition of the model:  states, names,
# possible transtions and waiting time distributions
states_1 <- c("1","2","3")
mtrans_1 <- matrix(FALSE, nrow = 3, ncol = 3)
mtrans_1[1, 2:3] <- c("W","W")
mtrans_1[2, c(1,3)] <- c("EW","EW")
mtrans_1[3, c(1,2)] <- c("W","W")

## Default initial values in a model without covariates
init_1 <- param.init(data = asthma, states = states_1, mtrans = mtrans_1)

## Definition of initial values in a model without covariates
init_2 <- param.init(data = asthma, states = states_1, mtrans = mtrans_1,
          dist_init=c(rep(1.5,6),rep(1.8,6),rep(2,2)),
          proba_init=c(0.2,0.8,0.3,0.7,0.35,0.65))

## Default initial values with a covariate "Sex"
# influencing transitions " 1->2" and "3->2"
init_3 <- param.init(data = asthma, cov=as.data.frame(asthma$Sex),
          states = states_1, mtrans = mtrans_1, cov_tra=list(c("12","32")))

## Definition of initial values with a covariate "Sex" 
# influencing transitions " 1->2" and "3->2"
init_4 <- param.init(data = asthma, cov=as.data.frame(asthma$Sex),
          states = states_1, mtrans = mtrans_1, cov_tra=list(c("12","32")),
          dist_init=c(rep(1.5,6),rep(1.8,6),rep(2,2)),
          proba_init=c(0.2,0.8,0.3,0.7,0.35,0.65), coef_init=rep(0.3,2))
          
init_5 <- param.init(data = asthma, cov=as.data.frame(asthma$Sex),
          states = states_1, mtrans = mtrans_1, cov_tra=list(c("12","32")),
          coef_init=c(0.2,0.5))          

## Definition of initial values without dataset in an illness-death model
## 1 - healthy, 2 - illness, 3 - death
states_2 <- c("1","2","3")
mtrans_2 <- matrix(FALSE, nrow = 3, ncol = 3)
mtrans_2[1,c(2,3)] <- c("E","W")
mtrans_2[2,c(1,3)] <- c("EW","EW")
init_6<-param.init(states=states_2, mtrans=mtrans_2, proba_init=c(0.7,0.3,0.2,0.8))