In the context of computers and computer systems, a backup is a copy of some data. This copy can be used when the original data is changed, or lost. Losing data is common: A 2008 survey found that two thirds of respondents had lost files on their home PC. Another purpose of backing up data is to have a copy that represents an earlier state of the data, before it was changed. Organisations may have rules which state how long data should be kept, and what kinds of data these rules apply to. In many countries there are rules that specify that certain kinds of data need to be kept for a given time. An example of this is the data used for accounting.