Once preferences have been harmonised, the next consideration is whether to apply formal approaches or deliberative approaches or both. There are a number of criteria that are relevant here. These should be considered as tentative criteria, as it is difficult to give clearcut criteria for choosing either formal or deliberative decision appraisal methods. Often both deliberative and formal methods may be equally relevant within a given adaptation decision. In order for formalisation to be appropriate a decision must be well-defined. A decision is well-defined and can be formalized under a following conditions:

• A decision among a set of options (also called alternatives, strategies, actions) has been identified. Notably, the identification of this set is not addressed by decisionanalytical methods.
• Outcomes of implementing options are known, that is they have been computed using either methods of risk assessment for present climate extreme event risks or residual impact projection methods for future climate (both slow onset and extremeevent). In the former case, outcomes may be expressed either probabilistically (with a likelihood of occurrence) or via scenarios (without a likelihood of occurrence). In the later case outcomes may only be represented via scenarios as probabilities cannot be associated to different pathways of socio-economic development and associated emission scenarios, which drive climate change and its impacts.
• Outcomes are characterised by one or several attributes (also called metrics, criteria, values), where at least one attribute describes the costs of planning and implementing an option.
• A baseline, which is a “do nothing” option against which to measure the values of the attributes can be established.

In general, further criteria to be fulfilled for formal decision appraisal to be appropriate are summarised in Table 2.7. These criteria relate to the feasibility and cost of formalizing a decision. Formalising a decision requires being able to translate the “real-world” complexity into the canonical form that formal methods rely on: one decision among a set of options (also called alternatives, strategies, actions) with each option being characterised by a set of attributes (also called metrics, criteria, values). The attributes describe both the costs of implementing an option as well as cost and benefits of the outcomes of implementing options. For decisions that are not well-defined and are interconnected to other decisions this might be difficult to do or the costs of information gathering and processing might be prohibitively high. It may then be appropriate to make individual decisions informally on the basis of intuition.

Empirical criteria	Formal appraisal	Intuitive/deliberative appraisal
Ambiguity on options, outcomes and baselines)	Low	High
Interconnectedness of decisions	Low	High
Information gathering and processing costs	Low	High
Importance of money in decision	High	Low
Experience on similar decisions with immediate feedback.	Low	High

Table 2.7: Criteria relevant to selecting formal or informal methods for appraising options.

Finally, a further important criteria for applying formal decision making methods is that they are often prescribed by the policy or legal context.