The failure of MTTF in availability evaluation

In this paper, a truncated bathtub is proposed to model the failure rate of a product with perfect burn-in. We expose some counter-intuition observations, e.g., increasing the MTTF does not necessarily increase the average and instantaneous availability during the product´s lifetime, and the average and instantaneous availability could be improved without changing the MTTF. The MTTF and the steady-state availability are not particularly informative and fail to evaluate the product with time-varying failure rate and short upgrading interval