The data in Table 1 are not real — nor is the "experiment" of burning the bulbs continuously. We made up this example to illustrate the exponential model described in this section. As we will discuss later, a more general model, the Weibull model, is usually used to model failure times for electrical and electronic components.
The basic assumption behind the exponential model is that the time to failure is independent of past history. Thus, the exponential model tends to underestimate the number of failures at startup and again toward the tail end of the distribution. Still, in some cases, it is not a bad approximation — particularly if failures on startup are excluded.