Interesting article in May 2006 Chemical Engineer by Chris Beale.
includes Donald Rumsfeld's famous quote "There are known knowns. These are things we know we know. There are known unknowns. That is to say, things we know that we don't know. But there also unknown unknowns. These are the things we don't know we don't know."
Chris points out that people often interpret risk assessment as certainties, when this often far from the case. He proposes three levels of uncertainty.
1. Using established technology. Minimal uncertainty.
2. Incremental change. Risks considered to be similar to those already known. Use management of change but recognise there may be incorrect assumptions about impact of change.
3. Step changes to completely new product or process. Need to pilot and then scale up. Expect to uncover many gaps in knowledge.
The suggested yard stick for assessments is to consider if you assessed the risk using worst case data, would it change your recommendations for risk reduction measures? If yes, uncertainty could be critical and a cautious approach is required.