Minimum sample size for the survey measurement of a wealth-dependent parameter with the UK VPF as exemplar

Measurement of an economic good by opinion survey constitutes a variant of the political opinion polls widely familiar from news reporting. The paper relates the minimum sample size needed for the survey measurement of a wealth-dependent parameter to the smallest sample for a political poll giving the same precision. Measuring a strongly wealth-dependent parameter by survey requires a sample size of ~2000 or more to provide precision equivalent to the 3% margin of error customary in UK political opinion polls. It is shown that the survey measurement of the "value of a prevented fatality" (VPF) used in the UK as a health and safety spending yardstick requires ~3000 people to be questioned. The analysis shows the actual sample size used, 167, to be inadequate. This adds to the problems besetting the UK VPF, as the method the surveyors used to interpret their data has already been shown invalid.