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A change in the level of inventory will not explain or measure the change in profit.
This simply calculates the level of revenue, which does not necessarily equal the change in income from one period to the next.
Under variable costing all fixed costs are expensed in the period in which they are incurred. Therefore, if the unit contribution margin is stable and the fixed costs are stable, the difference in income between two periods can be explained by change in the level of sales multiplied by the contribution margin per unit.
A change in the level of inventory will not explain or measure the change in profit.