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That if a mayonnaise packer and seller wants to know what percentage of the people in Pennsylvania prefers lemon flavored mayonnaise over other flavors, they don't have to ask everyone in Pennsylvania to find out! If they ask a representative sample of Pennsylvanians, say 1,500 to 3,000, which they prefer, they can project the results onto the entire population within a 'margin of error' of between 2.5 and 4%. That's close enough for most marketing planning purposes.
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