This is one of the most important questions to ask before beginning the prototyping process. The answer to this question may determine your choice of prototyping options. Below is a real-life situation where neglecting to ask this all-important question unfortunately resulted in a Career Limiting Decision (CLD).
The initial prototypes failed in mechanical testing. A subsequent redesign yielded parts that performed well. The designer, however, had prototyped the initial parts in an alloy and process that was not representative of the eventual production process. Without knowing the real cause of the part failure, the company decided to redesign the part to make it stronger. As a result of over-designing the part, the real expense revealed itself later when the unnecessary redesign added $9.00 to the production cost of each part. With an annual production rate of more than 1,000,000 parts, such an added cost is certainly an issue.