11.1 INTRODUCTION

The Total Time on Test ( TTT) concept, which was introduced by Barlow, Bartholomew, Bremner and Brunk (BBBB) in Chapters 5 and 6 of [1] in 1972, has proved to be a very useful tool in many reliability applications, such as

1. model identification [2],

2. aging properties analysis [2],

3. age replacement policy [3],

4. dependent failure data analysis [3],

and so on. In [4], Bergman illustrated how the TTT transform, a certain function of the survival function, and an empirical counterpart, called the TTT plot, can be used in connection with optimization problems for some burn-in models. Though some of these models may be optimized in a fairly straightforward way, there are several advantages with the TTT technique. One is the possibility for sensitivity analysis. Another is the possibility to apply a similar technique when the exact cdf is not known but a sample of observed times to failure is available.