Abstract
<title>Abstract</title> <p>Background: A lower prevalence of allergy in children has been observed across numerous studies in the farming environment. However, establishing a causal relationship is contentious due to the typical farm versus non-farm study design, which may have introduced a stratification collider bias. Methods: We conducted a quantitative meta-analysis of published risks from farming studies, categorizing them into subgroups based on representative or stratified sampling. The two risk ratios are subjected to another fixed effects meta regression model and the difference is quantified. Additionally, we analyzed the correlation across generations to indicate if either exposure or disease changes among children. Finally we report synthesis of migration studies and simulate outcomes under various disease prevalences. Results: The results indicate that the lower prevalence of allergic diseases in farming environments depends on the sampling method used. The studies with random sampling (N=6) provided an OR of 1.2 which is significantly different from the OR of 0.6 in studies with stratified sampling (N=24, P=0.0001 for difference). Children of diseased farmers are not inherently 'protected' at farms as the association is found in their parents as well. Simulating a healthy worker effect in the farming communities shows that the observed ORs can already be observed if 1 in 100 children per generation moves out of the farming environment. Conclusion: The three analytical approaches presented here collectively suggest that the currently available observational evidence is insufficient to establish a protective effect of farming.</p>