MMP.18:2 - Problem

Locally useful models can give an unusable combination. An exchange may be counted twice or disappear between different time steps. Components may share an unknown while their combined calculation treats its values as independent. An average may conceal a variation needed by a nonlinear response. A sequential software call may insert a delay into relations intended to hold together.

The difficulty is to construct the joint model without silently changing what the components mean or the answer they are meant to support. Compatibility at the interface and adequacy in the subject remain separate questions.