Considerable uncertainty is inherent in the willingness-to-pay (WTP) estimates made as part of monetary valuation.  Whilst part of this reflects inherent variations in individual preferences, much is also due to lack of consistency in methodology.  As a consequence, the range of values shown by inter-study comparisons typically dwarfs that found within individual studies.  This creates difficulties for attempts to derive generally applicable WTP estimates on the basis of previous studies. 

Various methods may be used to deal with these uncertainties.  Ideally, an informal meta-analysis should be used, as recommended by the European Commission (1995) and Friedrich and Bickel (2001).  In this, judgements are first made of the quality of individual studies, and these used to dermine which studies should be included.  Results from the selected studies are then used to construct interval ranges of unit values from the mean WTP reported in each case. 

This approach is only feasible where a sufficient range of suitable studies exists.  Where this is not the case, uncertainties can only be assessed more approximately. One approach is to impute standard deviations for the main health endpoints.  A study by the European Commission (2005), for example, suggested that a geometric standard deviation of 2 can reasonably be assumed for mortality impacts and chronic bronchitis, while a value of ~1.1 is appropriate for other, less severe, morbidity end-points with a cost of illness component of WTP.  These might therefore be applied more generally where other information is lacking.