The Answer is…

Define the claims you want to make for the product [the qualities, performance, reliability, etc.], detail how the product is to be designed to meet these claims by objective measurements, and then design exact tests to ensure you can make the measurements to demonstrate the claims.

The technical answer is that product quality, performance and reliability are DEFINED² , DESIGNED, TESTED and VALIDATED only by:

Defining MEASURABLE quality, performance and reliability REQUIREMENTS for the product.

Establishing Quality PROCESSES for development, production and maintenance.

Designing the product to the REQUIREMENTS within the quality PROCESS for development.

Testing, MEASURING and COMPARING the test results to the REQUIREMENTS.

Producing the product within the Quality PROCESS for production.

Testing, MEASURING and COMPARING the test results to the production PROCESS

Refining and repeating these processes over time to IMPROVE the quality, reliability and performance.

Within this list are two activities that are usually not included in a product development³ effort. If they ARE included then, more often than not, they are incomplete. These activities are (1) DEFINING what the product quality and Reliability need to be and (2) VALIDATING by test, measurement and comparison to what was DEFINED to validate what is claimed. The last activity in the list above, 'improving the quality and Reliability of the product/processes over time', is hardly ever done, except by large firms.

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² See: Product Engineering - What's Quality Got to do with it?, R. Haney
³ See: Good Product Engineering Practices, R. Haney