Development trials. Exploratory development trials used to improve a designer's understanding of how the product works are an essential part of the development process but have no value for validation. However, development trials that follow a protocol and are written up in a report may save work later. When early drafts of a specification include phrases like "to be decided," feasibility trials may show what performance levels can be expected — for example, the lifetime of a hinge, the number of operations before cleaning is needed, and the fluid pressure at the tip of a needle.

Design reviews Formal design reviews are conducted by the design team with assistance from others who can bring a fresh view and challenge assumptions. One meeting may be enough for a simple product but reviews for mechanical, electrical, software, and system design can take up many days. The design review considers the FMEA, development trials, design calculations, and decisions that have been made, all with a particularly critical eye.

Verification tests. These are the principal element of design validation. Some of these tests are performed on complete product, but others may be done on components or sub-assemblies. This is especially relevant to long-term reliability where a pump or motor may be tested in isolation to demonstrate that its lifetime is sufficient. The tests should include the effects of variation on performance, including variation which comes from tolerances in manufacture and from the product's environment. The variation can be allowed to occur naturally by using many people as test subjects, or it may be simulated by deliberate control of key characteristics, such as the viscosity of a drug or the storage temperature.

The test protocols and pass limits are approved before the tests commence. Any deviations from the protocol must be agreed upon, and a list of discrepancies or failures must be maintained. If the development work was thorough, there should be few discrepancies; the purpose of the tests is confirming that the performance is within specification.

Discrepancies can be resolved with design changes, but this must be accompanied by an analysis showing how other elements of the performance specification might be affected. In many cases, this analysis must be followed by repeating some verification tests. If the number of discrepancies and design changes increases, it may be necessary to redefine the verification tests as development trials and commence a new verification.

Some discrepancies can be resolved by simply amending the specification. In theory this should not happen if a top-down approach was taken in creating the specification. In reality, part of the performance specification often is written after feasibility studies, which were performed under ideal conditions on a laboratory bench. A test unit using production parts under "worst case" conditions can fail if the effects of the tolerances are not considered.

A third way to resolve discrepancies is identifying assignable causes of failure — the protocol was not followed, there was a power failure, a test lead broke. However, there is a risk that these causes become a series of excuses that are applied until a passing result is obtained.

Validation tests. Validation tests are performed when it is not possible to objectively measure performance. They are applied, for example, to the ease of assembly of a device, the legibility of labels, and the instruction manual. A team of about 10 people is selected with the requirement that they are representative of the intended users and are not familiar with the product. This excludes the design team and many engineers and managers in the quality, production, and marketing departments. The team follows the validation protocols and their subjective assessments are recorded and compared with the pass limits. The criteria for deviations and discrepancies that apply to verification tests also apply to validation.

Post-market surveillance. Information from real users also must be collected, but it is not part of the formal design validation for most products. (Clinical trials are a different matter entirely.) Questionnaires and interviews provide the best feedback about ease of use and operability and are used to confirm that the verification and validation tests were an effective surrogate for actual use.