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A functional structure within a business is when different operations, like payroll or IT, are separated out into duties designated to one person or group per duty, and a simple structure, commonly found in small businesses, is when the owner or the general manager handles the positions that would be separated out in the functional structure. They are similar because, although duties are devised in one form or completely handled through one source in the other, all designated duties to be done by a business are handled.
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