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What is "marginal analysis?"
"The study of variables in terms of the effects that would occur if they were changed by a small amount. For example, rather than analyse whether or not it is in the interest of an individual to spend money on food at all, attention can sensibly be focused on whether or not welfare could be enhanced by spending slightly more or less on food...
The margin is important in economics as it is the impact of small changes in variables rather than their level per se that determines whether rational economic agents change them.