Han, Peisong; Song, Peter X.-K. - In: Statistics & Probability Letters 81 (2011) 3, pp. 438-445
In some commonly used longitudinal clinical trials designs, the quadratic inference functions (QIF) method fails to work due to non-invertible estimation of the optimal weighting matrix. We propose a modified QIF method, in which the optimal weighting matrix is estimated by a linear shrinkage...