If the previous steps of issue framing and design have been properly done, and the assessment protocol is carefully followed, the execution phase ought to be relatively straightforward, and generate few surpises.  A number of difficulties and dangers may nevertheless arise.  A large and varied team of people may, for example, be involved in doing the assessment, and it can be difficult to ensure that these work together and retain a clear and common view of the agreed principles and goals.

Partly for this reason - but also because of the inevitable pull of individual interest and the push of the available data and models - assessments may drift from their original purpose, so that in the end they no longer satisfy user needs. 

In  addition, uncertainties may grow, unseen, as the analysis proceeds to the point where the results become of little value.  

In order to safeguard against these, a number of cross-cutting actions are essential throughout the execution phase:

1. Procedures and facilities for effective collaboration within the assessment team need to be maintained, for example via a collective on-line workspace and regular meetings, while wider liaison needs to be maintained with the stakeholders;
2. Strict procedures for documentation and tracking need to be enforced, to ensure that a full audit trail is maintained for the assessment, and is available for scrutiny at the end of the study;
3. A rigorous system of data management and quality assurance needs to be in place, to ensure that all data and methods are valid, and that the results make sense;
4. A continuous process of uncertainty monitoring and evaluation needs to be maintained, to give early warning of problems, and to enable corrective measures to be taken before uncertainties become excessive;
5. Repeated referral needs to be made to previous studies, to give reminders about unforeseen problems and to help avoid unnecessary duplication of work;
6. Methods are needed to assess the performance of the assessment, both in terms of meeting its objectives and its scientific credibility.

Links to guidance and tools to help achieve these are given in the panel to the left.