# Fit a semi-parametric joint model Fits a semi-parametric joint model as described by Liang et al. (2009). ```r Liang( data, Yname, Xnames, Wnames, Znames = NULL, formulaobs = NULL, id, time, invariant = NULL, lagvars = NULL, lagfirst = NULL, maxfu, baseline, Xfn = NULL, Wfn = NULL, ... ) ``` ## Arguments - `data`: data frame containing the variables in the model - `Yname`: character string indicating the column containing the outcome variable - `Xnames`: vector of character strings indicating the names of the columns of the fixed effects in the outcome regression model - `Wnames`: vector of character strings indicating the names of the columns of the random effects in the outcome regression model - `Znames`: vector of character strings indicating the names of the columns of the covariates in the visit intensity model - `formulaobs`: formula for the observation intensity model - `id`: character string indicating which column of the data identifies subjects - `time`: character string indicating which column of the data contains the time at which the visit occurred - `invariant`: a vector of variable names corresponding to variables in data that are time-invariant. It is not necessary to list every such variable, just those that are invariant and also included in the visit intensity model - `lagvars`: a vector of variable names corresponding to variables which need to be lagged by one visit to fit the visit intensity model. Typically time will be one of these variables. The function will internally add columns to the data containing the values of the lagged variables from the previous visit. Values of lagged variables for a subject's first visit will be set to NA. To access these variables in specifying the proportional hazards formulae, add ".lag" to the variable you wish to lag. For example, if time is the variable for time, time.lag is the time of the previous visit - `lagfirst`: A vector giving the value of each lagged variable for the first time within each subject. This is helpful if, for example, time is the variable to be lagged and you know that all subjects entered the study at time zero - `maxfu`: The maximum follow-up time per subject. If all subjects have the same follow-up time, this can be supplied as a single number. Otherwise, maxfu should be a dataframe with the first column specifying subject identifiers and the second giving the follow-up time for each subject. - `baseline`: An indicator for whether baseline (time=0) measurements are included by design. Equal to 1 if yes, 0 if no. - `Xfn`: A function that takes as its first argument the subject identifier and has time as its second argument, and returns the value of X for the specified subject at the specified time. - `Wfn`: A function that takes as its first argument the subject identifier and has time as its second argument, and returns the value of W for the specified subject at the specified time - `...`: other arguments to Xfn and Yfn ## Returns the regression coefficients corresponding to the fixed effects in the outcome regression model. Closed form expressions for standard errors of the regression coefficients are not available, and Liang et al (2009) recommend obtaining these through bootstrapping. ## Details This function fits a semi-parametric joint model as described in Liang (2009), using a frailty model to estimate the parameters in the visit intensity model The Liang method requires a value of X and W for every time over the observation period. If Xfn is left as NULL, then the Liang function will use, for each subject and for each time t, the values of X and W at the observation time closest to t. ## Examples ```r # replicate simulation in Liang et al. ## Not run: library(data.table) library(survival) datasimi <- function(id){ X1 <- runif(1,0,1) X2 <- rbinom(1,1,0.5) Z <- rgamma(1,1,1) Z1 <- rnorm(1,Z-1,1) gamma <- c(0.5,-0.5) beta <- c(1,-1) hazard <- Z*exp(X1/2 - X2/2) C <- runif(1,0,5.8) t <- 0 tlast <- t y <- t + X1-X2 + Z1*X2 + rnorm(1,0,1) wait <- rexp(1,hazard) while(tlast+wait