1.  Content

• In general, it is better to be more, rather than less, inclusive at the issue-framing stage since, if the assessment methodology is correct, factors which are not important in reality will lead to small effects, relative to the uncertainties involved, and thus contribute little to the central estimates of impact;
• Factors should not be omitted simply because of the lack of firm evidence or data, since this biases assessments to things that are already well-known and measured.  Instead plausibility of effect should be the primary criterion for inclusion.  Factors which are less well-evidenced, however, will introduce greater uncertainty into the assessment (and thus lead to a wide variation in the estimated impacts).

2.  Space

• Extending assessments over a larger area than necessary will increase the effort and cost involved, and may exacerbate problems of finding relevant data.  In proportional terms (e.g. where impacts are measured as a percentage change in mortality or cost), it may also lead to some dilution of impact - though where outcome indicators are measured in absolute terms (e.g. as excess mortality or cost) this will not be an issue.

• Assessments should not be constrained by artificial boundaries, which ignore significant transfers (e.g. of pollutants or affected people), since this will result in an under-estimation of the overall impact. Geographically, therefore, assessments should generally be based on ‘catchment areas’ or ‘zones of influence’, which reflect the full extent of the processes of concern. 

3.  Time

• Extending assessments over too long a period (e.g. too far into the future) will likewise increase difficulties in carrying out the assessment, and may increase substantially the uncertainties involved.   If the time-period for the assessment is unduly truncated, however, substantial under-estimation of the impacts may occur, by ignoring delayed and secondary effects.   

• On the whole, assessments should aim to cover the full life-cycle of any development (e.g. policy, technology), and at least the full life of those exposed to any risks (or likely to receive benefits) within that period.  Assessments should be extended wherever there is evidence for direct inter-generational effects (e.g. via reproductive health).