Resistance to alteration of data stored in a computer by system errors, so that only authorized and proper alteration occurs. Data integrity is a general concept with a number of different facets. Most obviously, it is a measure of the reliability of data read from magnetic media, in terms of the absence of undetected errors (see error rate). However , it is also a property of structured data (e.g. a database) that ensures that collections of data items are consistent with one another, particularly when copies of the same data are kept at different locations.