The first level of RE covers Qualitative Life Tests, which identify predominant failures and failure modes in functionality without attempting to make any predictions as to the product’s life under stated or implied use conditions. Qualitative tests are performed on a small number of samples with each sample subjected to a single severe level of stress, to a limited number of simultaneous stresses or to a time-varying stress (i.e. stress cycling, cold to hot, etc.). If the sample survives, it passes the test. Otherwise, appropriate actions need to be taken to improve the product's design in order to eliminate the cause of failure. Note that Qualitative Tests can be quite similar to Quality Control tests; however Qualitative Tests test functionality whereas Quality Control tests individual qualities. Information from these tests will be used for not only mitigating design flaws, but also for setting appropriate stress levels for follow-on accelerated life testing.

How is a quality related to a function? A function (action) may encompass one or more product qualities (attributes).

It’s critically important to note that if Qualitative Tests are not designed properly, they may cause the product to fail due to modes that would never be encountered during real use. So the Design Of Experiments (DOE) is imperative for establishing good Reliability tests. Qualitative Life Tests usually involve sophisticated test chambers and test equipment to perform stress pass/fail tests

The second level of RE covers Quantitative Life Tests, which are used to predict the life of the product (or, more specifically, life characteristics such as MTTF, B(10) life, warranty information, etc.) under normal use conditions, from data obtained during accelerated life tests. The data is statistically analyzed to produce reliability information that can include the determination of the probability of failure of the product under use conditions, mean life under use conditions and projected returns and warranty costs. It can also be used to assist in the performance of risk assessments, design comparisons, use under extreme conditions, etc.

Quantitative Tests also use sophisticated test equipment, but usually require a larger number of samples, very sophisticated multiple-stress tests, and statistical and probability analysis methodologies.

Note - To understand Reliability precisely one would need to learn about the basis of reliability, the ‘theory of measurement’ and the different types of measurement errors. Such errors play a key role in the validity of Reliability information. What one discovers, then, is that the certainty of Reliability can not be calculated; it can only estimated.