A larger positive index represents a wider entered price-cost margin. It does not automatically prove unlawful monopoly power or explain why the margin exists. Demand, fixed and sunk costs, product differentiation, regulation, innovation, capacity constraints and measurement error can all matter.
The U.S. Department of Justice has described the Lerner index as a short-run market-power measure while also warning that price-cost margins can be unsatisfactory evidence of monopoly power by themselves. Use it as one observation within a fuller competition analysis.
Document how marginal cost was estimated. Replacing it with average total cost changes the meaning, even if that number is easier to obtain from financial statements.
Cost-basis example: if the 12 USD figure includes fixed rent allocated per unit rather than the cost of producing one more unit, the resulting 0.400 is not a textbook Lerner index even though the arithmetic is correct. Estimating marginal cost may require operational or econometric evidence rather than a single income-statement line.
The measure can also change with output. A factory near capacity may face a high cost for the next unit, while the same firm at a lower output could face a different incremental cost. Prices may vary across customers or time as well. Record the product, market, quantity and period alongside the index. Comparisons are most meaningful when those definitions are aligned, and less meaningful when one observation uses a promotional price while another uses a list price.
