Parameter estimation based on set-membership approach is a non-probabilistic method for characterizing the uncertainty with which each model parameter is known. Only a class of uncertain static models containing several input-output relations is considered in this paper. Every equation error is bounded while parameters fluctuate inside a time-invariant domain represented by a zonotope. The proposed method helps to find the characteristics of this domain by taking the couplings related to bounded variables common to model output equations into account.