“Past performance is not necessarily indicative of future results” — The proven-in-use argument and the retrospective application of modern standards

The small print for most financial investment products on the market is introduced by the phrase "past performance is not necessarily indicative of future results". Despite this, the financial media\´s assessments of funds and fund managers are often based not on order book, prospects or qualifications but instead on their recent historical performance. These historical trends illustrate how performance fared against the various external stimuli faced in the recent past; but provide no guarantee of systematic behaviour or insight into how performance may be affected by new events. Given the well established difficulties in predicting the future based on the recent past, this paper explores the limitations of the "proven-in-use" argument for legacy Control and Instrumentation systems in nuclear power stations. Consideration is given to supplementing this with the retrospective application of modern standards to demonstrate the integrity of established equipment in a sector where future performance must be guaranteed, small print is irrelevant and stakeholders are not freely able to opt for an alternative.