AQA A-Level Economics (7136)

## Learning Outcomes

- Distinguish clearly between the **short run** and the **long run** in cost analysis
- Identify and calculate **fixed costs**, **variable costs**, **total cost**, **average costs**, and **marginal cost**
- Explain why **AFC** falls and why **AVC**, **AC**, and **MC** are usually U-shaped in the short run
- Analyse the relationship between **MC**, **AVC**, and **AC**
- Understand how **economies of scale**, **diseconomies of scale**, and **minimum efficient scale** affect long-run costs
- Apply cost formulas accurately in exam-style calculations and explanations

## AQA A-Level Economics (7136) Syllabus

For the AQA A-Level Economics (7136), you are expected to understand costs of production, with a focus on the following syllabus points:

- Know that the **short run** is a period in which at least one factor of production is fixed
- Know that the **long run** is a period in which all factors of production are variable
- Identify the difference between **fixed costs**, **variable costs**, and **total cost**
- Calculate **AFC**, **AVC**, **AC/ATC**, and **MC** using the correct formulas
- Explain why **AFC** always falls as output increases
- Understand why short-run **AVC**, **AC**, and **MC** are usually U-shaped
- Explain the relationship between **marginal cost** and **average cost curves**
- Understand **economies of scale**, **diseconomies of scale**, and **minimum efficient scale (MES)** in the long run

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between the short run and the long run in economics?
2. How do fixed costs differ from variable costs?
3. Why does average fixed cost fall as output rises?
4. What is marginal cost, and how is it calculated?
5. Why must marginal cost cut average cost at the minimum point of the average cost curve?

## Introduction

Costs of production are central to microeconomics because they help explain how firms make output decisions, how market structure develops, and why some firms can survive while others cannot. In exams, this topic often appears in definitions, calculations, data-response questions, and diagram-based analysis.

You need to know not just the formulas, but also the reasoning behind the cost curves. Strong answers explain why costs change as output changes, and they keep the **short run** and **long run** clearly separate.

> **Key Term: costs of production**
> The expenses a firm incurs when using factors of production and raw materials to produce goods or services.

### Short run and long run: the starting point

The first distinction to secure is between the **short run** and the **long run**. This is not mainly about clock time. It is about whether factors of production can be changed.

> **Key Term: short run**
> The time period in which at least one factor of production is fixed and cannot be varied.
>
> **Key Term: long run**
> The time period in which all factors of production are variable and the scale of production can be changed.

In the short run, a firm may be stuck with a given factory size, number of machines, or shop premises. It can still raise output by using more labour or raw materials, but at least one factor remains fixed.

In the long run, the firm can change everything. It can move to a larger site, install more machinery, redesign its production process, or fully alter capacity. This matters because short-run and long-run cost curves are shaped by different forces.

A very common mistake is to say the short run is "a few months" and the long run is "many years". That is too vague. A bakery might treat six months as the short run if it cannot move premises, while a software firm may change scale much faster.

> **Test Tip:** If an exam asks you to distinguish the short run from the long run, define them in terms of **fixed and variable factors**, not time length alone.

### Fixed, variable and total costs

In the short run, costs are split into **fixed** and **variable** costs.

> **Key Term: fixed cost**
> A cost that does not change with output in the short run.
>
> **Key Term: variable cost**
> A cost that changes as output changes.
>
> **Key Term: total cost**
> The total cost of production, equal to fixed costs plus variable costs.

The key formula is:

- **TC = TFC + TVC**

Examples of fixed costs in the short run include:

- rent on premises
- insurance
- salaries of senior permanent staff
- some advertising commitments

Examples of variable costs include:

- raw materials
- hourly wages for production workers
- packaging
- fuel directly linked to output

Fixed costs are still paid even if output is zero. Variable costs usually fall to zero if production stops completely.

A student error is to think fixed costs never change. They do change eventually, but not in the short run. That is why the long run has no fixed costs in economic theory: all costs become variable once the firm can change all inputs.

Here is a simple worked example.

A firm has fixed costs of £200. If it produces 50 units and variable costs are £300:

- **TC = TFC + TVC**
- **TC = 200 + 300**
- **TC = £500**

If output rises to 80 units and variable costs rise to £420, then:

- **TC = 200 + 420**
- **TC = £620**

The fixed cost stays the same, but total cost rises because variable cost rises.

> **Key Term: total fixed cost (TFC)**
> The total cost of fixed factors of production in the short run.
>
> **Key Term: total variable cost (TVC)**
> The total cost of variable factors of production.

### Average costs and marginal cost

Average costs are per-unit costs. They are often tested in multiple-choice and calculation questions.

> **Key Term: average fixed cost (AFC)**
> Fixed cost per unit of output, calculated as total fixed cost divided by output.
>
> **Key Term: average variable cost (AVC)**
> Variable cost per unit of output, calculated as total variable cost divided by output.
>
> **Key Term: average cost (AC/ATC)**
> Total cost per unit of output, calculated as total cost divided by output.
>
> **Key Term: marginal cost (MC)**
> The addition to total cost resulting from producing one extra unit of output.

The formulas are:

- **AFC = TFC / Q**
- **AVC = TVC / Q**
- **AC = TC / Q**
- **AC = AFC + AVC**
- **MC = ΔTC / ΔQ**

Because fixed costs do not change with output, marginal cost can also be found from the change in variable cost:

- **MC = ΔTVC / ΔQ**

Worked example:

A firm has:

- output = 20 units
- total fixed cost = £100
- total cost = £140

Step 1: Find average cost

- **AC = TC / Q**
- **AC = 140 / 20**
- **AC = £7**

Step 2: Find average fixed cost

- **AFC = TFC / Q**
- **AFC = 100 / 20**
- **AFC = £5**

Step 3: Find average variable cost

- **AVC = AC - AFC**
- **AVC = 7 - 5**
- **AVC = £2**

Now suppose total cost rises from £140 to £175 when output rises from 20 to 30.

Step 4: Find marginal cost

- **MC = ΔTC / ΔQ**
- **MC = (175 - 140) / (30 - 20)**
- **MC = 35 / 10**
- **MC = £3.50 per unit**

Students often divide total cost by output and call that marginal cost. That is wrong. Marginal cost is based on the **change** in cost, not the total.

AFC always falls as output rises because the same fixed cost is spread over more units. This is called **spreading overheads**. By contrast, AVC and AC usually fall at first, then rise.

> **Test Tip:** In calculation questions, write the formula first. This reduces errors and makes method marks more likely if you slip on arithmetic.

### Why short-run cost curves have their usual shapes

In standard theory, the **AFC** curve slopes continuously downward. It never becomes negative and approaches zero as output becomes very large.

The more demanding part is explaining the U-shape of **AVC**, **AC**, and **MC** in the short run.

The reason is the **law of diminishing marginal returns**. When more variable inputs are added to a fixed factor, workers may initially become more productive because of specialisation and better use of existing capacity. But beyond some point, the fixed factor becomes a constraint. There is less space, fewer machines per worker, or more congestion. Marginal product then falls.

When marginal product is rising, the cost of producing an extra unit tends to fall, so **MC** falls. Once diminishing marginal returns set in, each extra worker adds less output, so the cost of each extra unit rises, and **MC** rises.

This is why the short-run **MC** curve is usually U-shaped.

The same logic affects **AVC** and then **AC**:

- **AVC** may fall at first because variable inputs are being used more efficiently
- later, **AVC** rises because diminishing returns make each unit more costly
- **AC** also tends to be U-shaped because it reflects both falling AFC and changing AVC

At low output, falling AFC and falling or stable AVC pull AC down. At higher output, rising AVC outweighs the continuing fall in AFC, so AC rises.

The key curve relationships are:

- if **MC < AC**, then **AC is falling**
- if **MC > AC**, then **AC is rising**
- therefore **MC cuts AC at the minimum point of AC**
- similarly, **MC cuts AVC at the minimum point of AVC**

This relationship is mathematical, not just visual. If the extra unit costs less than the current average, it pulls the average down. If it costs more than the average, it pushes the average up.

Common student errors here include:

- saying AC rises because fixed costs rise with output
- confusing diminishing returns with diseconomies of scale
- claiming MC cuts AC at the highest point rather than the minimum point

### Long-run average cost, economies of scale and MES

In the long run, all factors are variable. So the focus shifts from diminishing returns to **scale**.

> **Key Term: long-run average cost (LRAC)**
> The lowest possible average cost of production at each level of output when all factors of production are variable.
>
> **Key Term: economies of scale**
> Reductions in long-run average cost as the scale of production increases.
>
> **Key Term: diseconomies of scale**
> Increases in long-run average cost as the scale of production becomes too large.

Long-run average cost may fall because larger firms can gain:

- **technical economies** from more efficient machinery
- **managerial economies** from specialist managers
- **purchasing economies** from bulk buying
- **financial economies** from cheaper borrowing
- **marketing economies** from spreading advertising costs
- **risk-bearing economies** from diversification

But growth is not always beneficial. Very large firms may suffer:

- communication problems
- weaker coordination
- slower decisions
- more layers of management
- lower worker motivation

That is why the **LRAC** curve is often shown as U-shaped: first falling due to economies of scale, then rising due to diseconomies of scale. However, you should know that the long-run curve does not have to be perfectly U-shaped in every industry. In some industries it may flatten out for a long range of output.

> **Key Term: minimum efficient scale (MES)**
> The lowest level of output at which a firm fully or nearly fully exploits economies of scale and reaches minimum long-run average cost.

MES matters for market structure. If minimum efficient scale is small relative to the size of the market, many firms can operate efficiently. If MES is very large relative to market size, only a few firms may be able to survive, which can lead to a concentrated market.

For example, local cafés may have a low MES, so many firms can compete. Heavy industries such as aircraft production may have a much higher MES, limiting the number of efficient firms.

> **Test Tip:** When evaluating economies of scale, do not just list them. Explain the mechanism. For example, bulk buying lowers unit input costs, so average cost falls.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The **short run** is defined by at least one fixed factor of production
- The **long run** is defined by all factors of production being variable
- In the short run, **total cost = total fixed cost + total variable cost**
- **Fixed costs** do not vary with output in the short run, while **variable costs** do
- **AFC = TFC / Q**, **AVC = TVC / Q**, and **AC = TC / Q = AFC + AVC**
- **Marginal cost** is the extra cost of producing one more unit: **MC = ΔTC / ΔQ**
- **AFC** always falls as output rises because fixed costs are spread over more units
- In the short run, **AVC**, **AC**, and **MC** are usually U-shaped because of **diminishing marginal returns**
- If **MC** is below **AC**, average cost falls; if **MC** is above **AC**, average cost rises
- **MC** cuts both **AVC** and **AC** at their minimum points
- In the long run, **economies of scale** reduce LRAC, while **diseconomies of scale** increase LRAC
- **Minimum efficient scale** is the lowest output at which long-run average cost is minimised

## Key Terms and Concepts

- costs of production
- short run
- long run
- fixed cost
- variable cost
- total cost
- total fixed cost (TFC)
- total variable cost (TVC)
- average fixed cost (AFC)
- average variable cost (AVC)
- average cost (AC/ATC)
- marginal cost (MC)
- long-run average cost (LRAC)
- economies of scale
- diseconomies of scale
- minimum efficient scale (MES)


Production, Costs and Revenue - Costs of production