5.41 There is a wide range of uncertainty that affects interventions, but in appraisal it is often due to lack of evidence or understanding of the likely impact of new interventions. Research and evidence from evaluations of previous interventions, pilot studies and experience of "what works" can help to reduce this uncertainty. The following paragraphs set out a range of techniques for dealing with uncertainty in appraisal.
5.42 As used in the Green Book, risk and optimism bias are closely linked but distinct concepts, for more detail on methods see Annex 5.