“…In practice, q ( t ) is often prespecified to balance the computational convenience and model flexibility. The choices used in the literature include the Box‐Cox transformation function, the simple linear function q ( t )= t , the so‐called “general” function , and a rich class of functions . In this paper, we use a two‐dimensional function As will be shown in the following, this q ( t ) function allows fast computation and usually works well for smooth functions through Taylor expansion approximations.…”