Authored study notes for the Marshall elasticity practice clip.
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Topic: price elasticity of demand (PED)
Definition: PED = percent change in quantity demanded / percent change in price
Source named in the lecture: Alfred Marshall
Worked number: PED 2.5 means a 1 percent price rise cuts quantity demanded by 2.5 percent
Rule: demand is elastic when |PED| > 1
Review flag: check the sign and the units before quoting the number