Form Validation is the process where the data entered in a form is verified against a set of rules. If it is incorrect or data is missing, the interface requests that a specific field must be fixed (partial validation) or the full form be resubmitted with correct information (full validation).
Sometimes errors are unavoidable filling a form even though they are not expected. When an error happens messages are used to describe the problem, including information on how to fix and prevent said error. To keep these messages user-friendly, validation messages in forms should: Describe the nature of the problem, using an honest and clear language. Reduce the possibility of making errors by providing examples. Shown just in time, e.g., a real-time message while the user is typing informing valid format rules. Prevent reiteration, describing how to fix the error committed. Be reachable, showing them where the errors took place.
• Cooper, A., Reimann, R. and Cooper, A. (2007). About face 3.0. Wiley.
• Wroblewski, L. (2008). Web form design. Rosenfeld Media.