“…However, it is sometimes more realistic to assume that the life distri bution of a repaired item is different from the life distribution of a new item; e.g., it may be stochastically less. It has been assumed by some authors (e.g., Blumenthal, Greenwood, and Herbach [4], Brown and Proschan [5], Balaban and Singpurwalla [1], and Block, Borges, and Savits [3]) that a failed item, on repair, may return to a "good as old" state (imperfect repair). Thus, if the original life distribution of the item when it was new is F, then the item upon repair wilj^ have survival function F, where t is its age at failure and F,(x) = F(t + x)/F(t).…”