## Core Concepts in Marginal Costing
### Four Definitions You Must Distinguish
Marginal Cost
The incremental cost of producing one additional unit. Measured by total variable costs attributable to that unit.
Marginal Costing
A costing system (technique) where products, services, and inventories are valued at variable costs only. Fixed costs are excluded from product valuation.
Differential Cost
The difference in total costs between two different production levels. It is relational — it shows how cost changes when moving from Level A to Level B.
Incremental Cost
The increase in costs due to a change in volume or process. Key distinction:
- Marginal cost = change in cost for exactly one extra unit
- Incremental cost = change in cost for one unit or a batch/volume
> Marginal cost is a special case of incremental cost.
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### Characteristics of Marginal Costing
| Characteristic | What it means |
|---|---|
| Classification of Costs | All costs split into fixed and variable; semi-variable costs further bifurcated |
| Cost Treatment | Only variable costs (DM + DL + Variable FOH) form the product cost |
| Inventory Valuation | Finished goods and WIP valued at marginal (variable) cost only |
| Fixed Cost Treatment | Treated as period costs — charged to P&L of the period incurred, not carried in stock |
| Price Determination | Prices set with reference to marginal cost + contribution margin |
| Profitability Analysis | Judged by contribution margin (Sales − Variable Cost), not net profit per unit |
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### Important Facts About Marginal Costing
1. Not a distinct costing method — it is a technique used alongside job costing, process costing, standard costing, etc.
2. Cost ascertainment is based on cost behaviour (fixed vs variable), not functional classification.
3. Preferred for decision making over absorption costing because it isolates variable cost behaviour and avoids distortion from fixed cost apportionment.
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### Period Cost
Costs not assigned to products but charged as expenses against revenue of the period incurred. In marginal costing, all fixed costs (manufacturing and non-manufacturing) are period costs.
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### Advantages vs Limitations
Advantages
1. Simplified pricing — marginal cost is constant per unit
2. No under/over-recovery of overheads
3. Realistic profit reporting (fixed costs not deferred in stock)
4. Facilitates break-even and CVP analysis
5. Better expenditure control via fixed/variable split
6. Supports make-or-buy, discontinuation, and replacement decisions
7. Enables short-term profit planning using BEP charts
Limitations
1. Difficult to precisely classify costs as fixed or variable
2. Contribution alone is insufficient without considering key/limiting factors
3. Risk of selling below full cost if sales staff misunderstand marginal cost
4. Incorrect WIP valuation in large contracts (fixed overhead excluded)
5. Linear cost assumptions may not hold in practice
6. Ignores time factor and investment magnitude
7. Stocks and WIP are understated compared to absorption costing