## 3. Strategical Approach  
Most prototypes and/or test samples during the product development are restricted to being used in several tests, such as a various design conformity check, performance requirement test, or other performance- or design-related general tests. These V&V tests play a key role in making important decisions, such as whether to continue or stop the product development project, make design changes, manage yields, redesign or optimize processes, and plan for process control. The decisions that are influenced by the results of V&V tests are often regarded as more critical than those related to a reliability assessment or improvement. In addition, a reliability test generally incurs high costs and takes a long time. Therefore, the reliability test is often given lower priority.  
Moreover, the additional reliability test is required for a newly developed product without prior reliability information, because conducting an AT without prior information can result in very poor accuracy of the reliability information; that is, more prototypes may be consumed for estimating the reliability information.  
However, with an increasing focus on product liability laws and safety concerns, there is a growing demand for high levels of product reliability. Therefore, a strategic approach that can efficiently conduct a multiple purpose test and estimate the prototype reliability with a small and limited number of test units and a shorter duration of test is required in modern industry.  
The strategic approach is as follows: During the product development period, the number of prototypes is limited, and test samples must be efficiently allocated for use in multiple purpose tests, which include various design conformity checks, performance requirement tests, and other performance- and design-related general tests and environmental tests (e.g., HTOL) shown in Figure 2. The goal is to maximize the utility of the test samples and extract as much reliability information as possible, even with limited or no prior information. In the product development stage, most V&V tests are conducted  
Figure 1. Stress loading method.  
under normal use conditions, but some tests are conducted under harsh condition, such as the HTOL and HTSL. If the HTOL stress level is considered the maximum stress level of ATs, the requirement of conducting PADTs can be satisfied. Therefore, multiple purposes are satisfied by conducting V&V tests as usual and gathering degradation data to obtain reliability information simultaneously.  
Figure 3 shows an example of the structure of the test time under the constant-stress loading method. The V&V tests are conducted until (a), under the normal use condition, as they generally require a short test duration. An additional test continues until (d) in order to gather the degradation data, since the amount of degradation at (a) is insufficient to analyze the product reliability. At an HTOL stress level, the HTOL test is completed at (b), but the test continues until (c) to collect the reliability data that minimize the asymptotic variance of the model parameters. In other words, there are data from two levels of degradation: one from the normal use condition and one from the severe condition; the maximum likelihood estimation method can be used to estimate the parameters of the degradation path model. Two more factors to consider when developing a strategic approach are the stress loading scheme and the degradation path model. Depending on the type of stress loading method, the test procedures are slightly different. The degradation path model provides the background of the numerical results.