The High-Low Method Mistake That Costs ACCA MA Candidates Marks

The high-low method looks like one of the easier techniques in ACCA Management Accounting, which is exactly why it catches so many candidates out. The mechanics are short enough to learn in five minutes: find the variable cost per unit from the change in cost between two activity levels, then use that to find the fixed cost. The trouble is that the method only works if you select the right two points, adjust for any change in the underlying cost structure between them, and then build the resulting cost equation correctly. Miss any one of those three steps and the numbers still come out, they just come out wrong, and a wrong-but-confident answer scores worse than a clearly labelled uncertain one.

What The High-Low Method Is Actually Doing

The high-low method splits a semi-variable cost, one with both a fixed and a variable element, into its two components using only two data points. The formula for variable cost per unit is the change in total cost divided by the change in activity level between the highest and lowest points. Once you have the variable cost per unit, you substitute it back into either point's total cost to isolate the fixed cost. It is a two-step calculation built on a simple assumption: that cost behaves in a straight line across the range of activity you are examining.

Mistake One: Selecting Points By Cost Instead Of Activity

The method's name causes the first mistake. 'High-low' refers to the highest and lowest levels of activity, machine hours, units produced, orders processed, not the highest and lowest cost figures. In most textbook examples the highest activity level and the highest cost happen to be the same row of data, so candidates get away with sloppy selection for years without noticing the error. Then a real exam question includes a data set where the two do not line up, usually because of a one-off cost, a discount, or efficiency gains at higher volumes, and candidates who instinctively scan for the biggest and smallest cost figures pick the wrong pair.

Here is a data set where that gap shows up.

The highest cost in this table is Month 1, at 24,500, but Month 1 does not have the highest activity level. The highest activity level is Month 4, at 9,000 units. The lowest activity level is Month 3, at 4,000 units, which also happens to have the lowest cost, so half the trap is hidden.

A candidate who wrongly pairs the highest cost, Month 1 at 24,500 for 5,000 units, with the lowest cost, Month 3 at 19,000 for 4,000 units, would calculate: variable cost per unit equals 24,500 minus 19,000, divided by 5,000 minus 4,000, which is 5,500 divided by 1,000, equals £5.50 per unit. Substituting back: fixed cost equals 24,500 minus (5.50 times 5,000), equals 24,500 minus 27,500, a negative £3,000. A negative fixed cost is a strong signal that the wrong pair of points has been used; fixed costs cannot sensibly be negative, and if your own answer produces one, that is worth stopping and rechecking before you move on.

The correct pair, by activity level, is Month 4, 9,000 units at £23,000, and Month 3, 4,000 units at £19,000. Variable cost per unit equals 23,000 minus 19,000, divided by 9,000 minus 4,000, which is 4,000 divided by 5,000, equals £0.80 per unit. Fixed cost equals 23,000 minus (0.80 times 9,000), equals 23,000 minus 7,200, equals £15,800, which checks out against the low point too: 19,000 minus (0.80 times 4,000) equals 19,000 minus 3,200, equals £15,800.

Mistake Two: Ignoring A Step Change In Fixed Costs

The high-low method assumes fixed costs stay constant across the whole range between your two chosen points. In practice that assumption breaks whenever a business adds capacity partway through the range, a second supervisor once output passes a threshold, an extra machine rented once volume exceeds a certain level, additional warehouse space once stock holding rises. If a step change like this sits between your low point and your high point, the raw high-low calculation blends a genuine variable-cost increase with a fixed-cost jump, and the resulting variable cost per unit is wrong.

Take a business with a low point of 2,000 units costing £9,000, and a high point of 7,000 units costing £19,500. Suppose the scenario also tells you that a second supervisor was hired once output passed 5,000 units, adding £2,000 of fixed cost per month from that point on.

Ignoring the step change, a candidate would calculate: variable cost per unit equals 19,500 minus 9,000, divided by 7,000 minus 2,000, which is 10,500 divided by 5,000, equals £2.10 per unit. That figure is contaminated, because part of the 10,500 increase in cost is really the £2,000 fixed-cost step, not variable cost responding to extra units.

The correct approach strips the step out of the high point before calculating, so both points reflect the same fixed-cost structure. Adjusted high-point cost equals 19,500 minus 2,000, equals £17,500. Variable cost per unit equals 17,500 minus 9,000, divided by 7,000 minus 2,000, which is 8,500 divided by 5,000, equals £1.70 per unit. Base fixed cost, below the 5,000-unit threshold, equals 9,000 minus (1.70 times 2,000), equals 9,000 minus 3,400, equals £5,600. Above the threshold, total fixed cost becomes £5,600 plus the £2,000 step, £7,600. Checking against the high point: £7,600 plus (1.70 times 7,000) equals 7,600 plus 11,900, equals £19,500, which matches the original figure.

Mistake Three: Mixing Up Variable Cost Per Unit With Total Cost

The final mistake happens after the hard part is already done correctly. Having found a variable cost per unit and a fixed cost, some candidates then build the forecast cost equation incorrectly, either forgetting to multiply the variable cost per unit by the new activity level, forgetting to add the fixed cost back in at all, or presenting the variable cost per unit itself as if it were the answer to a 'what will total cost be' question.

Using the figures from the step-change example above, the correct cost equation above the 5,000-unit threshold is: total cost equals £7,600 plus (£1.70 times activity in units). Forecasting cost at 10,000 units: total cost equals 7,600 plus (1.70 times 10,000), equals 7,600 plus 17,000, equals £24,600. A candidate who confuses variable cost per unit with total cost might simply write £1.70, or multiply 1.70 by the wrong quantity, or omit the fixed cost entirely and answer £17,000. Every one of those answers throws away marks that the earlier, correct calculation had already secured.

A Short Checklist Before You Submit The Answer

• Have you selected your two points by activity level, not by cost?
• Does the scenario mention any capacity change, new equipment, extra staffing or similar step change between your two points, and if so, have you adjusted for it before calculating variable cost per unit?
• Does your fixed cost figure make sense, positive, and roughly consistent whichever point you substitute back into?
• When forecasting cost at a new activity level, have you multiplied variable cost per unit by that new activity level and then added fixed cost, rather than presenting either figure on its own?

The high-low method sits alongside other cost-behaviour assumptions that ACCA MA tests throughout the syllabus, including the assumptions behind breakeven analysis assumptions, so it is worth treating cost estimation and breakeven technique as a linked pair of topics rather than isolated calculations. Getting the pairing of points, the step-change adjustment and the final equation right, every time, turns a two-minute calculation from a common source of lost marks into a reliable source of full marks.