Common Mistakes

Common Mistakes: Cost Accounting

The Cost Accounting chapter demands care in classifying costs, arranging the Manufacturing Account and analysing the Break-even Point. The common mistakes below help you avoid the numerical and sequencing slips that often cost marks.

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Classifying and calculating cost components

Mistake: mixing direct costs with overhead, e.g. putting the factory supervisor's salary or machine depreciation into Direct Labour. Fix: remember that only costs traceable directly to one unit of output (raw materials, production workers' wages, direct expenses such as the hire of a special machine) are direct costs; every indirect factory cost is grouped under Overhead.
Mistake: calculating Direct Material Cost from purchases alone. Fix: use Opening raw material + Purchases + Carriage inwards − Closing raw material. Carriage inwards (the cost of bringing raw materials to the factory) must be added, not treated as overhead or ignored.
Mistake: treating Prime Cost as if it were the Cost of Production. Fix: Prime Cost = Direct Material + Direct Labour + Direct Expenses only. Overhead and the Work-in-Progress adjustment are added afterwards to arrive at the Cost of Production.

Opening and closing Work-in-Progress (WIP)

The Manufacturing Account and transfer of cost

Mistake: putting selling or administrative expenses (e.g. salesmen's commission, office rent) into the Manufacturing Account. Fix: the Manufacturing Account carries only factory/production costs; selling and administrative expenses go to the Income Statement. This is why some costs go to the Statement of Cost of Production and others to the Income Statement.
Mistake: forgetting to transfer the Cost of Production to the Trading Account. Fix: the Cost of Production must be transferred (it replaces 'purchases' for a manufacturer) into the Trading Account to work out gross profit. Without this transfer the accounts are incomplete.
Mistake: placing items on the wrong side of the 'T' form, e.g. recording closing stock on the debit side. Fix: in the 'T' form, costs incurred (materials, labour, overhead, opening WIP) go on the debit; closing WIP and the Cost of Production transferred out go on the credit, so both sides balance.

Fixed cost, variable cost and contribution margin

Break-even calculation and graph

Correct order of the Statement of Cost of Production (example)

Item	RM
Opening raw materials	10,000
Purchases of raw materials	50,000
Add: Carriage inwards	2,000
Less: Closing raw materials	8,000
Direct Material Cost	54,000
Direct Labour Cost	30,000
Direct Expenses	6,000
Prime Cost	90,000
Overhead Cost	20,000
	110,000
Add: Opening Work-in-Progress	5,000
	115,000
Less: Closing Work-in-Progress	7,000
Cost of Production	108,000

Note the order: direct materials first, then Prime Cost, Overhead, add Opening WIP and subtract Closing WIP to reach the Cost of Production.

Break-even calculation (Contribution Margin Method)

Item	Value
Selling price per unit (RM)	50
Variable cost per unit (RM)	30
Contribution margin per unit (RM)	20
Total fixed cost (RM)	100,000
Break-even Point (units)	5,000
Target profit (RM)	40,000
Quantity for target profit (units)	7,000

Contribution Margin per unit = 50 − 30 = 20. BEP = 100000 ÷ 20 = 5000 units. For target profit RM40000: (100000 + 40000) ÷ 20 = 7000 units.

What is the difference between Prime Cost and Cost of Production?

Prime Cost is the total of direct costs only, namely Direct Material + Direct Labour + Direct Expenses. Cost of Production is Prime Cost + Overhead, then adjusted by adding Opening WIP and subtracting Closing WIP. Many candidates stop at Prime Cost and wrongly label it as Cost of Production.

Where do Opening and Closing WIP go in the Cost of Production calculation?

After obtaining Prime Cost + Overhead, add Opening Work-in-Progress (part-finished work from the previous period completed now) and subtract Closing Work-in-Progress (unfinished work carried to the next period). Reversing these add/subtract signs is the most frequent mistake.

Why does contribution margin use only variable cost and not total cost?

Contribution margin is the surplus of revenue after covering variable cost, and this surplus is what contributes towards covering fixed cost and then earning profit. Hence Contribution Margin per unit = Selling Price per unit − Variable Cost per unit. Fixed cost is deducted afterwards, and the Break-even Point is reached when total contribution margin equals total fixed cost.

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