Internal Validity. The authors implemented the contracts in the HW system,
which is a threat to internal validity. However, we minimize this threat since we
followed the HW requirements documentation, which contains information with respect to pre- and postconditions. Hence, we did not infer any contract (e.g. a precondition). We implemented all contracts according to the documentation.
As explained, this was one of the main reasons to use the Health Watcher system in
this study.