Roberto this is an excellent topic of discussion and as you will find in the field, often a topic of contention. The FDA provides guidelines on how tests should be executed. Take a look at the recent changes to the guidelines for LAL testing and you will see that the FDA does not claim specifics. Another example would be the guidelines to establish cleanroom monitoring. Fortunately there are ISO standards that come and serve the need of the company. By keeping abreast with the ISO/ASME standard, the company is able to mitigate risk and have some guidance by the subject matter expert in that field. Once enough literature is collected, the skilled engineer should initialize a design of experiments or protocol based on the information they have that pertains to their product.