The Need for Formal Evidence Synthesis in Food Policy: A Case Study of Willingness-to-Pay

SIMPLE SUMMARY: Meta-analysis is a statistical technique used to combine results of different studies on the same topic. These methods are gaining increasing popularity in the field of consumer behaviour; however, the issue of publication bias, which is the tendency for the study outcome to influence whether the research is published or not, has received little attention in this field. This research, therefore, looked to explore publication bias in willingness-to-pay, using the example of willingness-to-pay for farm animal welfare. A systematic search of four online databases led to the inclusion of 54 studies for analysis. Publication bias was assessed by using four different tests. The results indicate that publication bias is present in the literature, with willingness-to-pay for farm animal welfare being overestimated as a result. Given the use of willingness-to-pay in policy, stakeholders should ensure that results of meta-analysis are assessed for publication bias. ABSTRACT: Meta-analysis is increasingly utilised in the understanding of consumer behaviour, including in relation to farm animal welfare. However, the issue of publication bias has received little attention. As willingness-to-pay (WTP) is widely used in policy, it is important to explore publication bias. This research aimed to evaluate publication bias in WTP, specifically public WTP for farm animal welfare. A systematic review of four databases yielded 54 studies for random effects meta-analysis. Publication bias was assessed by the Egger test, rank test, contour-enhanced funnel plots, and the Vevea and Hedges weight-function model. Results consistently indicated the presence of publication bias, highlighting an overestimation of WTP for farm animal welfare. Stakeholders should be wary of WTP estimates that have not been critically evaluated for publication bias.