Flukey It sounds more like Cost Benefit Analysis (CBA) exercise you are undertaking, which has a large element of subjectivity, rather than a 'reasonably practicable' one. The whole concept of RP is a subjective one and reminds me of a quote from recent post - 'Not everything that can be measured counts, and not everything that counts can be measured'. Good luck. Ray

I think that Ray sums it up quite well. I think that most of us do try to measure this although highly subjectively and often without recording our reasoning; merely the outcome that we have decided. Of interest to some, is that ‘benefit’ is increasingly featuring in the consideration of RP in particular recreational and sports activities, where the cost, the risk AND the benefit of taking part are all considered. Some RAs are straightforward and the ‘argument’ of RP is relatively easy to decide and justify (even if at a later date). However, I have done some more ‘complex’ assessments where I have ‘teased’ out the various issues, the different approaches that could be taken and the arguments for and against each one before reaching a conclusion about the suggested approach taken and the reasoning for doing so. However, in the example you use, while you have said that the cost is significant and that the ‘worst case’ is death, you have said little about the LIKELIHOOD of the outcome being realised (and that in itself can be highly subjective). In contrast to JJ, I would say that to spend tens of millions of £'s to (potentially) save one life, might well NOT be considered RP. For the most part, it is something of a conundrum that we all have to deal with, and I am afraid that there are very often no straightforward answers. At the end of the day we all have to make the call and if necessary justify this at some point in the future. Sometimes the judge/jury will agree, sometimes they won't; such is the beauty of our legal system. Sorry I can’t be more helpful. Crack on!

Thanks canopener...your last couple of paragraphs are exactly where I am. In terms of LIKELIHOOD of outcome we are talking UNLIKELY as a worst case.

An approach you could try is a 'reverse CBA' - in other words having estimated the costs of additional controls and the 'benefits' of avoiding one fatality, work out the highest frequency of occurrence that would justify such a spend. That may help you decide 'it's obviously going to be much more/less often than that' and thus aid the decision making. As with any QRA, you also need to check the sensitivity of your calculation(s) to the assumptions you make. E.g. if each of your estimates/guesses is wrong by (say) an order of magnitude, does that change the final decision? Another approach is to use the guide published by Step Change in Safety for decision-making in areas of uncertainty - a diagram that indicates what relative weight to give to: custom & practice; published standards; QRA; etc. linked to the novelty or otherwise of the hazards/risks you are trying to assess. Also check out HSE's list of typical ways QRA can be misused and think how to demonstrate you aren't doing so, remembering that a really clever QRA specialist can probably use it to justify almost any decision, if a client wishes him/her to do so! Hope these thoughts help in addition to the good points made by others.