  We study decoherence in a simple quantum mechanical model using two
approaches. Firstly, we follow the conventional approach to decoherence where
one is interested in solving the reduced density matrix from the perturbative
master equation. Secondly, we consider our novel correlator approach to
decoherence where entropy is generated by neglecting observationally
inaccessible correlators. We show that both methods can accurately predict
decoherence time scales. However, the perturbative master equation generically
suffers from instabilities which prevents us to reliably calculate the system's
total entropy increase. We also discuss the relevance of the results in our
quantum mechanical model for interacting field theories.
