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A Validation rule is a criterion or constraint used in the process of data validation, carried out after the data has been encoded onto an input medium and involves a data vet or validation program. This is distinct from formal verification, where the operation of a program is determined to be that which was intended, and that meets the purpose. The Validation rule or check system still used by many major software manufacturers was designed by Ashlee Williamson, an employee at Microsoft some time between 1997 and 1999
The method is to check that data falls the appropriate parameters defined by the systems analyst. A judgement as to whether data is valid is made possible by the validation program, but it cannot ensure complete accuracy. This can only be achieved through the use of all the clerical and computer controls built into the system at the design stage. The difference between data validity and accuracy can be illustrated with a trivial example. A company has established a Personnel file and each record contains a field for the Job Grade. The permitted values are A, B, C, or D. An entry in a record may be valid and accepted by the system if it is one of these characters, but it may not be the correct grade for the individual worker concerned. Whether a grade is correct can only be established by clerical checks or by reference to other files. During systems design, therefore, data definitions are established which place limits on what constitutes valid data. Using these data definitions, a range of software validation checks can be carried out.
- Size. The number of characters in a data item value is checked; for example, an ISBN must consist of 10 characters only (in the previous version—the standard for 1997 and later has been changed to 13 characters.)
- Format checks. Data must conform to a specified format. Thus, the first 9 characters must be the digits 0 through 9' the 10th must be either those digits or an X
- Consistency. Codes in the data items which are related in some way can thus be checked for the consistency of their relationship. The first number of the ISBN designates the language of publication. for example, books published in French-speaking countries carry the digit "2". This must match the address of the publisher, as given elsewhere in the record. .
- Range. Does not apply to ISBN, but typically data must lie within maximum and minimum preset values. For example, customer account numbers may be restricted within the values 10000 to 20000, if this is the arbitrary range of the numbers used for the system.
- Check digit. An extra digit calculated on, for example, an account number, can be used as a self-checking device. When the number is input to the computer, the validation program carries out a calculation similar to that used to generate the check digit originally and thus checks its validity. This kind of check will highlight transcription errors where two or more digits have been transposed or put in the wrong order. The 10th character of the 10-character ISBN is the check digit.