Table 2 Some sources for differences in cost‐of‐illness studies.

Topic	Differences
Perspective of costs	Societal perspective (all costs regardless of who pays, ie, direct and indirect costs)
	Payer perspective (only costs that are covered by the specific payer, generally only direct costs)
Definition of sample	Population sample (representative of the entire patient population in a geographic area)
	Specific sample (eg, by disease severity, by age, by point of care)
Definition of relevant costs	Consumption of patients with RA (all consumption of a patient with the disease)
	Consumption for RA (only RA‐related care and consumption)
Mode of data collection	Top‐down (from national statistical databases) vs bottom‐up (from medical charts and patients)
	Prospective (following a sample of patients over time) vs retrospective (consumption in the past 1, 3, 12 months)
Valuation	Tariffs (costs fixed by an insurance)
	Opportunity costs (costs in their next best alternative)