While we may use Coca Cola and Pepsi to demonstrate goods that are typically seen as substitutes (elastic), we can only assert this in the broad sense, or across a society of consumers. While the majority of consumers likely believe that these two products are perfect substitutes, there are a number of consumers who may feel just the opposite. For example, in Atlanta, Georgia (Coca Cola's Headquarters), many consumers simply will not buy/drink Pepsi because of their loyalty to Atlanta's own. Therefore, these consumers do not see Coca Cola as an elastic good with respect to the price of Pepsi; these consumers will not alter their behavior based on the price of Pepsi.
We find this situation in a number of goods that can typically be described as substitutes when talking about them in a general sense. This is where individual taste and preferences come in; many consumers are loyal to their favorite brands.
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