The three stages of production describe how total, average and marginal product change as a firm adds more of a variable input—usually labor—while holding other inputs fixed. Stage I ends at maximum average product; Stage II ends at maximum total product, where marginal product is zero; Stage III begins when marginal product turns negative and total output falls.
What the three stages describe
This is a short-run model of input and output, not a sequence of calendar-time steps in making a product. In the standard setup, a firm varies one input while at least one other input, such as factory space or machinery, remains fixed. The framework is commonly taught as the law of variable proportions. SATHEE’s NCERT-based chapter on production and costs uses labor to explain the product measures.
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- Total product (TP): the total quantity of output produced.
- Average product (AP): output per unit of the variable input, such as output per worker.
- Marginal product (MP): the additional output attributable to one more unit of the variable input.
How the three stages differ
| Stage | Total product (TP) | Average product (AP) | Marginal product (MP) | Boundary |
|---|---|---|---|---|
| I: Increasing returns to the variable factor | Rises; it may first rise at an increasing rate and later at a diminishing rate. | Rises. | Positive; it may rise at first and then fall. | Ends at maximum AP, where MP equals AP. |
| II: Diminishing returns | Rises at a diminishing rate. | Falls but remains positive. | Positive and falling. | Ends at maximum TP, where MP is zero. |
| III: Negative returns | Falls. | Remains positive while output and input remain positive. | Negative. | Begins after MP falls below zero. |
Where the stage boundaries fall
Stage I ends at maximum average product
AP rises while MP is greater than AP. AP reaches its maximum when MP equals AP; after that, MP is below AP and AP declines. That crossing marks the end of Stage I.
Stage II ends at maximum total product
As long as MP is positive, another unit of the variable input adds output, so TP continues to rise. When MP reaches zero, TP is at its maximum. This is the Stage II/Stage III boundary.
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Stage III begins when marginal product is negative
Once MP becomes negative, adding another unit of the variable input reduces total output. TP then falls. The Institute of Company Secretaries of India’s Business Economics Study Material also identifies the zero-MP point as the maximum of TP.
How to identify the stages on a graph
- Plot TP, AP and MP against the quantity of the variable input.
- Find the maximum of AP, where MP crosses AP. The range before this point is Stage I.
- Find the maximum of TP, where MP reaches zero. The range between the two maxima is Stage II.
- Label the range after MP falls below zero as Stage III, where TP declines.
The key distinction is between a falling marginal product and falling total output. In Stage II, each additional input unit adds less output than the previous one, but its contribution is still positive, so total output keeps rising. Output falls only after MP becomes negative.
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Why the distinction matters
The stage names describe how product measures behave as the variable input changes while other inputs are fixed. In particular, “diminishing returns” does not mean that production has already fallen: it refers to declining additional output in Stage II. For a fuller treatment of the production-and-costs topic, see the Class XII introductory economics textbook available from SUSTech’s repository.
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