compute_sigma function

Compute covariance matrix for some reference-based methods (JR, CIR)

Adapt covariance matrix in reference-based methods. Used for Copy Increments in Reference (CIR) and Jump To Reference (JTR) methods, to adapt the covariance matrix to different pre-deviation and post deviation covariance structures. See Carpenter et al. (2013)

compute_sigma(sigma_group, sigma_ref, index_mar)

Arguments

• sigma_group: the covariance matrix with dimensions equal to index_mar for the subjects original group
• sigma_ref: the covariance matrix with dimensions equal to index_mar for the subjects reference group
• index_mar: A logical vector indicating which visits meet the MAR assumption for the subject. I.e. this identifies the observations that after a non-MAR intercurrent event (ICE).

References

Carpenter, James R., James H. Roger, and Michael G. Kenward. "Analysis of longitudinal trials with protocol deviation: a framework for relevant, accessible assumptions, and inference via multiple imputation." Journal of Biopharmaceutical statistics 23.6 (2013): 1352-1371.