This paper addresses the question of how the errors in the yield and recovery values calculated by Equations 1 and 2 are related to the underlying uncertainty in the raw assay data. This is a non-trivial question because the response from the two-product formula is nonlinear and has a singularity. analysed the problem by using the general expression for estimating the variance Vg of a function of three variables g(x,y,z) calculated from the partial derivatives and variances of the three variables (Vx, Vy, Vz):