In this paper, we consider the problem of how and when to lump subsystems together inlarge linear interconnected systems with external inputs and outputs. The motivation for this work isthat we often want to reduce the model order of large interconnected systems, but before we do thatwe should identify what interconnection structures that are worth preserving in the reduction. For thispurpose a constrained Hankel-norm is introduced in this paper, and a so-called lumping index is derivedfrom it. A subsystem that is not a very independent subsystem in the interconnected system has a largelumping index, and we argue in the paper that it then is a good candidate for lumping. As an example,a large mechanical spring-mass system is considered.