  We consider the frequency response problem and derive a posteriori error
estimates for the discrete error in a reduced finite element model obtained
using the component mode synthesis (CMS) method. We provide estimates in a
linear quantity of interest and the energy norm. The estimates reflect to what
degree each CMS subspace influence the overall error in the reduced solution.
This enables automatic error control through adaptive algorithms that determine
suitable dimensions of each subspace. We illustrate the theoretical results by
including several numerical examples.
