 Because this model involves giving issues away to unspecific populations, the statistics only entail the number of issues distributed, and not who reads them.This is the model used by many trade magazines (industry-based periodicals) distributed only to qualifying readers, often for free and determined by some form of survey.Because of costs (e.g., printing and postage) associated with the medium of print, publishers may not distribute free copies to everyone who requests one (unqualified leads); instead, they operate under controlled circulation, deciding who may receive free subscriptions based on each person's qualification as a member of the trade (and likelihood of buying, for example, likelihood of having corporate purchasing authority, as determined from job title).