As health care embraces the tenets of evidence-based medicine it is important to ask questions about how evidence is produced and interpreted. This essay explores normative dimensions of evidence production, particularly around issues of setting the tolerable level of uncertainty of results. Four specific aspects are explored: what health care providers know about statistics, why alpha levels have been set at 0.05, the role of randomization in the generation of sufficient grounds of belief, and the role of observational studies. The essay concludes with recommendations to acknowledge the value permeation of outcome measures and suggests that attention to reasoning and argument analysis can augment traditional evidence-based approaches in providing a robust critical approach to medical knowledge.