Inventory investment is a relatively minor component of GDP, but we need to understand it in some detail because it plays a key role in the Keynesian approach. When a firm produces output, it does one of two things with it: it either sells it or adds it to inventory. Thus an accounting relationship within a firm is that
production = sales + changes in inventory.
If a firm produces more than it sells, its stocks of inventories increase. If a firm sells more than it produces, its stocks of inventories decrease. The inventories that a firm holds are counted as part of its capital stock, so any change in firms’ inventories is counted as a component of investment.
Suppose General Motors (GM) produces 10 million cars, anticipating that it will sell them all. Then imagine that demand is lower than expected, so it only sells 9.9 million. The result is that 100,000 cars pile up on GM’s lots, and the GM accountants record this as an addition to inventory. We want GDP to measure both production and spending, but we have 100,000 cars that have been produced but not purchased. The national income accounts get around this problem by effectively pretending that GM bought the cars from itself.
If the cars are then sold in the following year, they will not contribute to GDP in that year— quite properly, since they were not produced that year. The national accounts in the next year will show that 100,000 cars were sold to households, but they will also show that inventories decreased by 100,000 cars. Thus the accounts record expenditures on these cars as part of durable goods consumption, but the accounts also contain an offsetting reduction in inventory investment.
In some cases, firms change their stocks of inventory as a part of their business strategy. More often, changes in inventories occur because a firm did not correctly forecast its sales. Unplanned inventory investment is an increase in inventories that comes about because a firm sells less than it anticipated. Because GM expected to sell all 10 million cars but sold only 9.9 million, GM had 100,000 cars of unplanned inventory investment.
Moreover, GM is likely to react swiftly to this imbalance between its production plans and its sales. When it sees its sales decrease and its inventory increase, it will respond by cutting its production back until it is in line with sales again. Thus, when an individual firm sees inventories increase and sales decrease, it typically scales down production to match the decrease in demand. Now let us think about how this works at the level of an economy as a whole. Suppose we divide total spending in the economy into unplanned inventory investment and everything else, which we call planned spending.
Toolkit: Section 16.19 "The Aggregate Expenditure Model"
Planned spending is all expenditure in the economy except for unplanned inventory investment:
GDP = planned spending + unplanned inventory investment.
This equation must always hold true because of the rules of national income accounting.
Begin with the situation where there is no unplanned inventory investment—so GDP equals planned spending—and then suppose that planned spending decreases. Firms find that their production is in excess of their sales, so their inventory builds up. As we just argued, they respond by decreasing production so that GDP is again equal to planned spending, and unplanned inventory investment is once again zero. Thus, even though unplanned inventory investment can be nonzero for very short periods of time, we do not expect such a situation to persist. We expect instead that actual output will, in fact, almost always equal planned spending.
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