The slope of the PPF and opportunity cost is different at every point    The opportunity cost of food in terms of housing rises as more food is produced. Moving from point A to point B means 40 units of housing must be given up to gain 40 more units of food. Moving from point B to point C means giving up 40 more units of housing to gain only 20 units of food. When considering the trade-off required to move from point A to B we are tempted to say the opportunity cost is 1, and it is, on average, but the opportunity cost along a curved PPF is, in fact, different at each point. The slope at any point on the PPF represents the opportunity cost at that point.

   At point A the slope = 2. When it is producing 140 houses and 40 food units, an increase of food production by 2 units requires a reduction of housing output of 1 unit. At point B, the slope = .75, meaning that when housing output is 100 and food output is 80, housing output must be reduced by 1 unit if to obtain an increase in food production of .75 units. Once the society is a point C, where food output is 100 and housing output is 60, the slope = .25. Food production is already near its maximum; further increases in food output are very costly, requiring the society to give up 4 houses for 1 added unit of food.

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