Abstract: Some recent modeling results on burn-in are given. Results for mixed populations are discussed and burn-in at the system and component levels for coherent systems is studied. In particular, conditions are given on the underlying distribution so that the mixed distribution satisfies a strong type of aging property. Furthermore the questions of in what order or whether or not to burn-in the components and the system are discussed.
Key words and phrases: Burn-in, bathtub failure rate, mixed populations, reverse regular of order 2 (RR2), exponential family, cost function.