Your results probably won t look as smooth as the figures. The bathtub curve with premature wearout and retirement Figure 2. Unfortunately, no family of distribution functions fits these failure rate functions, so you should look at nonparametric estimates. Both failure rate functions in the figures show retirement. Figure 1 shows retirement following onset of wearout, and figure 2 shows retirement before wearout.
Retirement means fewer operating hours per calendar time unit.
This causes failure rate functions to decrease. One would prefer the product with the failure rate function in figure 2, because he wouldn t have to deal with wearout. However, the product may cost too much, so either figure may describe products. If the failure rate function isn t constant, maintenance should be age specific. If the failure rate function is decreasing, leave the product alone, unless it breaks.