**Linearization method**

In the estimation of the variance of a general non-linear estimator, we adopt the method based on the so-called Taylor series expansion. The method is usually called the linearization method because we first reduce the original non-linear quantity to an approximate linear quantity by using the linear terms of the corresponding Taylor series expansion, and then construct the variance formula and an estimator of the variance of this linearized quantity. See page 141 in Lehtonen, R. & Pahkinen, E. (2004)