AP Microeconomics Unit 3: Production, Cost, and the Perfect Competition Model Notes

Introduction

A firm compares marginal benefit with marginal cost, but the correct decision also depends on the time horizon: fixed inputs shape short-run production, avoidable costs shape shutdown, and all costs matter for long-run entry and exit.

Short-run production holds at least one input fixed. As variable input is added, specialization may initially raise marginal product, but crowding a fixed input eventually lowers it. When the variable input has a constant price, that falling marginal product appears as rising marginal cost.

A firm maximizes profit at the output where the last unit adds at least as much revenue as cost and the next unit would not. Profit can still be negative there. Competitive entry and exit then connect one firm’s decision to the market without treating market quantity and firm quantity as the same scale.

Variable input with a fixed input

marginal product

Marginal product and input price

marginal cost

Explicit and implicit costs

economic profit

Marginal revenue and marginal cost

profit-maximizing output

Price, AVC, and ATC

produce, shut down, enter, or exit

Market price

competitive firm demand

Current exam guidance: Aligned to the current 2026–27 AP Microeconomics framework. Every model identifies axes, assumptions, movement or shift, market structure, and the quantitative or welfare consequence.

3.1

The Production Function

How does adding labor change output when another input is fixed?

Short answer

Total product rises by each worker’s marginal product; diminishing marginal returns begins when that added output starts falling, even while total output continues to rise.

Know firstScarcity · Marginal analysis · Short run and long run

Read the change in output, not just its level

A short-run production function holds at least one input fixed and varies another. In the schedule below, the first worker adds 12 units and the second adds 15. That increase can reflect specialization while the fixed workspace and equipment remain underused. Marginal product is the change in total product caused by one more unit of the variable input, while average product is output per unit of that input.

Diminishing marginal returns begins with the third worker because marginal product falls from 15 to 12. Total product still rises from 27 to 39, so diminishing returns does not mean negative returns or falling total output. Only a negative marginal product would make another worker reduce total output. The schedule is a physical relationship for fixed technology and inputs; it says nothing about a worker’s worth, wage, or effort.

Economic model

Total product can rise while marginal product falls

Synchronize total, marginal, and average product so the onset of diminishing marginal returns is visible without confusing a falling increment with falling output.

Held constant: Workspace, equipment, technology, worker-hours, and output quality are fixed; labor is the only changing input.
Time horizon: Short run, because at least one productive input is fixed.
Read the change: Moving from one labor level to another moves along this production function. Technology or a different fixed-input stock creates a new function.

Diminishing returns begins: worker 3

Total productMarginal productAverage product

Horizontal axis: Labor input
Vertical axis: Total product
Secondary axis: Product per worker

MP falls after its worker-2 peak while TP continues rising

MP_L=\frac{\Delta TP}{\Delta L},\qquad AP_L=\frac{TP}{L}

Total output rises from zero to 51 as labor rises from zero to five. Marginal product peaks at 15 for worker two, then falls to 12, 8, and 4 while total product keeps rising.

Read the model as a table

Total product can rise while marginal product falls values
Labor	Total product	Marginal product	Average product
0	0	—	—
1	12	12	12
2	27	15	13.5
3	39	12	13
4	47	8	11.75
5	51	4	10.2

Rows of workers operating lathes and other machinery across the Western Electric Tarheel Army Missile Plant floor in 1953. — A fixed plant changes what another worker can add
In the short run, the plant and its machine capacity are fixed while labor can change. Early workers may divide tasks more effectively; after the easiest gains are used, limited machines and floor space can make each added worker increase output by less.
Use this visual to see
See many workers share finite machines and floor space—the physical condition behind diminishing marginal returns.
What it cannot show
The photograph does not report output at different employment levels, marginal or average product, or the point at which diminishing returns begins.

Audit a production schedule

Subtract adjacent total-product rows to find each marginal product.
Divide total product by labor to find average product.
Locate the first decline in marginal product.
Check whether total product is still rising.
Name the fixed input and technology before generalizing.

Keep these three ideas

Marginal product is the change in total product.
Diminishing returns can occur while total product is rising.
The production function is conditional on fixed inputs and technology.

Production, Cost, and the Perfect Competition Model