we ascertained the states in which each firm collected sales taxes on online sales. For each firm, we followed the following sequence: First, we visited the firm’s website and searched for a listing of states in which tax was collected; second, if the website data was inconclusive, we contacted the firm’s customer service department; third, if customer service was unable or unwilling to provide the information, we researched the firm’s website, its Securities and Exchange Commission filings, and other public data, for a list of states in which the firm in has a retail store or other physical presence.