CREDIT CARD EMBLEM PRESENT versus NO CREDIT CARD EMBLEM on the tip tray is the Independent Variable (IV) in McCall & Belmont. The experimenter arranged by random assignment that some patrons got their bill on a tip tray with a Visa logo on it and other patrons just got a plain tip tray. This is the independent variable since it is the causal variable the experimenters can control and want to examine to see how it causes tipping. The independent variable is independent of the participant's behavior. It is totally up to the experimenter which customer gets which tip tray and nothing the customer does effects whether or not he or she gets a tip tray with or without the logo.
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