Comments on the Graphical Method

Although the graphical method is simple, it is quite laborious. Furthermore, many issues surrounding its use require careful consideration. Some of these issues are presented next:

What happens when no failures are observed at one or more stress level? In this case, plotting methods cannot be employed. Discarding the data would be a mistake since every piece of life data information is important. (In other words, no failures at one stress level combined with observed failures at other stress level(s) are an indication of the dependency of life on stress. This information cannot be discarded.)
In the step at which the life-stress relationship is linearized and plotted to obtain its parameters, you must be able to linearize the function, which is not always possible.
In real accelerated tests the data sets are small. Separating them and individually plotting them and then subsequently replotting the results, increases the underlying error.
During initial parameter estimation, the parameter that is assumed constant will more than likely vary. What value do you use?
Confidence intervals on all of the results cannot be ascertained using graphical methods.

The maximum likelihood estimation parameter estimation method described next overcomes these shortfalls, and is the method utilized in ALTA.