﻿## When using this code, please cite this article: de Bekker-Grob, E.W., Donkers, B., Jonker, M.F. et al. Patient (2015) 8: 373. https://doi.org/10.1007/s40271-015-0118-z## ############################################################################################################################################################################ ############################################################################################################################################################################ test_alpha=0.05 z_one_minus_alpha<-qnorm(1-test_alpha) test_beta=0.2 z_one_minus_beta<-qnorm(1-test_beta) parameters<-c(1.23 , -0.31 , -0.21 , -0.44 , 0.028 , -1.10 , -0.04 , -0.0015) ncoefficients=8 nalts=3 nchoices=16 # load the design information design<-as.matrix(read.table("D:\\Design_matrix_Illustration_DCE_Osteoporosis_treatment.txt",header=FALSE)); #compute the information matrix # initialize a matrix of size ncoefficients by ncoefficients filled with zeros. info_mat=matrix(rep(0,ncoefficients*ncoefficients), ncoefficients, ncoefficients) # compute exp(design matrix times initial parameter values) exputilities=exp(design%*%parameters) # loop over all choice sets for (k_set in 1:nchoices) { # select alternatives in the choice set alternatives=((k_set-1)*nalts+1) : (k_set*nalts) # obtain vector of choice shares within the choice set p_set=exputilities[alternatives]/sum(exputilities[alternatives]) # also put these probabilities on the diagonal of a matrix that only contains zeros p_diag=diag(p_set) # compute middle term P-pp’ middle_term<-p_diag-p_set%o%p_set # pre- and postmultiply with the Xs from the design matrix for the alternatives in this choice set full_term<-t(design[alternatives,])%*%middle_term%*%design[alternatives,] # Add contribution of this choice set to the information matrix info_mat<-info_mat+full_term } # end of loop over choice sets #get the inverse of the information matrix (i.e., gets the variance-covariance matrix) sigma_beta<-solve(info_mat,diag(ncoefficients)) # Use the parameter values as effect size. Other values can be used here. effectsize<-parameters # formula for sample size calculation is n>[(z_(beta)+z_(1-alpha))*sqrt(S??)/delta]^2 N<-((z_one_minus_beta + z_one_minus_alpha)*sqrt(diag(sigma_beta))/abs(effectsize))^2 # Display results (required sample size for each coefficient) N