If a test appears to be valid to participants or observers, it is said to have face validity. Face validity is one among many parameters used to assess the value of an experiment or test, and to gather information about how the experiment was conducted, and how applicable the results will be. However, it is not a perfect measurement, as assessments of face validity are very much based on personal experience; what seems valid and reasonable to one person may seem weak to another.
A classic use for this parameter is in the construction of a test or experiment. A researcher who wants to test something in an experimental situation would devise a test, and then ask for review from peers to determine whether or not the test is ethical, and whether or not the test will actually measure what the creator thinks it measures. For example, a researcher who wanted to test vocabulary of young children might propose showing children flashcards with terms and asking the children to define them, and some people might agree that the experiment has face validity, because it would appear to measure vocabulary skills in the test subjects.
The issue can get extremely complex with more nuanced experiments. The more detailed and complex the experiment, the trickier it can be to determine face validity. Experts in a field, for example, may say that the test is not valid, while laypeople may have the test explained to them and think that it sounds entirely reasonable. This is one reason why people are expected to submit proposed experiments for review by a group of their peers, and to request peer review for their results before publication, so that experts can weigh in on the situation.
Several things are considered when evaluating an experiment for face validity. The first is a simple measure of whether or not it will do what it is supposed to do. Consistency and reliability are also concerns; reviewers want to know that the results are reliable, and also that the results are repeatable. If a test protocol cannot be repeated, the results of the experiment will be regarded as questionable by the scientific community.
Asking for an evaluation of face validity is always a good idea. Having a fresh pair of eyes examine a protocol or planned test can point out flaws and issues which might not be apparent to the creator. Review also gives creators a chance to get feedback before they start, which can allow a test creator to adjust the test before it starts or to specifically address particular concerns in the writeup of the results.