across all points in time is by deleting (or overwriting) past generations (except the oldest and the most-recent-n generations) when necessary in a weighted-random fashion. For each deletion, the weight assigned to each deletable generation corresponds to the probability of it being deleted. One acceptable weight is a constant exponent (possibly the square) of the multiplicative inverse of the duration (possibly expressed in the number of days) between the dates of the generation and the generation preceding it. Using a larger exponent leads to a more uniform distribution of generations, whereas a smaller exponent leads to a distribution with more recent and fewer older generations. This technique probabilistically ensures that past generations are always distributed across all points in time as is desired. The weighted random method only has an advantage over a more systematic approach, when backups are irregular or missed. Incremented media method This method has many variations and names. A set of numbered media is used until the end of the cycle. Then the cycle is repeated using media numbered the same as the previous cycle, but incremented by one. The lowest numbered tape from the previous cycle is retired and kept permanently. Thus one has access to every backup for one cycle and to one backup per cycle before that. This method has the advantage of ensuring even media wear, but requires a schedule to be precalculated. See also Off-site data protection Legal hold References Tape-based computer storage Backup