Lerner index

The Lerner index, formalized in 1934 by British economist of Russian origin Abba Lerner, is a measure of a firm's market power.

Definition

The Lerner index is defined by:

$L={\frac {P-MC}{P}}$

where P is the market price set by the firm and MC is the firm's marginal cost. The index ranges from 0 to 1. A perfectly competitive firm charges P = MC, L = 0; such a firm has no market power. An oligopolist or monopolist charges P > MC, so its index is L > 0, but the extent of its markup depends on the elasticity (the price-sensitivity) of demand and strategic interaction with competing firms. The index rises to 1 if the firm has MC = 0.

The following factors affect the value of the Lerner index:

• the price elasticity of demand for goods produced by the company — the smaller the fluctuations in demand under the influence of prices, the smaller the elasticity and the greater the value of L;
• the interaction with competitors — the more of them and the larger their size, the less the company's ability to maximize profits and the smaller the L;
• the degree of regulation — the more actively the state conducts an antitrust policy, the lower the value of L.

The Lerner Rule or Lerner Condition is that if it is to maximize its profits, the firm must choose its price so that the Lerner Index equals -1 over the elasticity of demand facing the firm (note that this is not necessarily the same as the market elasticity of demand):

${\frac {P-MC}{P}}={\frac {-1}{E_{d}}}$

A drawback of the Lerner Index is that while it is relatively easy to observe a firm's prices, it is quite difficult to measure its marginal costs. In practice, the average cost is often used as an approximation.

The Lerner index can never be greater than one. As a result, if the firm is maximizing profit, the elasticity of demand facing it can never be less than one in magnitude (|E|<1). If it were, the firm could increase its profits by raising its price, because inelastic demand means that a price increase of 1% would reduce quantity by less than 1%, so revenue would rise, and since lower quantity means lower costs, profits would rise. Put another way, a monopolist never operates along the inelastic part of its demand curve.

Derivation

The Lerner Rule comes from the firm's profit maximization problem. A firm choosing quantity Q facing inverse demand curve P(Q) and incurring costs C(Q) has profit equalling revenue (where R = PQ) minus costs:

$Profit=P(Q)Q-C(Q)$

Under suitable conditions (that this is a convex maximization problem, e.g. P(Q) and C(Q) are linear functions), we can find the maximum by taking the derivative of profit with respect to Q and getting the first-order-condition:

${\frac {dProfit}{dQ}}=({\frac {dP}{dQ}}Q+P)-{\frac {dC}{dQ}}=0$

which gives the standard rule of MR = MC. To get the Lerner Rule, switch to the notation dC/dQ = MC and rewrite as

$P-MC=-{\frac {dP}{dQ}}Q$

Divide by P to get

${\begin{aligned}{\frac {P-MC}{P}}&=-{\frac {dP}{dQ}}{\frac {Q}{P}}\\&\\&=-1/\left({\frac {dQ}{dP}}{\frac {P}{Q}}\right)\\&\\&=-{\frac {1}{E_{d}}},\end{aligned}}$

using the derivative definition of elasticity.

Examples

Calculation

Let's suppose we need to fill in the gaps in the following table:

Industry	Price,P	Marginal cost,MC	Elasticity of demand,Ed	Lerner index
A	10			0.4
B	30		-2
C	40	30

Lerner index of the Czech credit market

Lerner index of the Czech credit market

Year	Mean	Median
2000	0.4438	0.5486
2001	0.5598	0.5455
2002	0.5723	0.5530
2003	0.6165	0.6089
2004	0.5430	0.5371
2005	0.5980	0.6163
2006	0.5673	0.5457
2007	0.5650	0.5739
2008	0.5109	0.5029
2009	0.3921	0.3289
2010	0.6742	0.6582

During the period 2000–2005 the Lerner index computed for the credit market slightly increased. It means that there was a slight decrease in competition. Then, during 2006–2009, there was a decrease in the Lernex index. In 2010 the Lerner index significantly increased. The mean of the Lerner index computed for the full sample is 53.58 %, which do not confirm either monopoly or perfect competition in the credit market of Czech Republic.

Lerner index of the Czech deposit market

Lerner index of the Czech deposit market

Year	Mean	Median
2000	0.5905	0.6898
2001	0.6864	0.6959
2002	0.6930	0.6692
2003	0.7170	0.7204
2004	0.6595	0.6482
2005	0.6955	0.7100
2006	0.6718	0.6806
2007	0.6696	0.6776
2008	0.6255	0.6328
2009	0.5266	0.4880
2010	0.7642	0.7355

This table represents the estimation of the market power in the deposit market in Czech Republic. During the period 2005–2009 the Lerner index computed for the deposit market slightly decrease, which means a slight increase in competition. In 2010, there was an increase in the Lerner index. It signals a decrease in competition. However, the value of Lerner index haven't changed significantly in estimated 10 years.

See also

• Ramsey problem
• Amoroso–Robinson relation
• Herfindahl Index
• Market segmentation index

References

• Lerner, A. P. (1934). "The Concept of Monopoly and the Measurement of Monopoly Power". The Review of Economic Studies. 1 (3): 157–175. doi:10.2307/2967480. JSTOR 2967480.
• https://journal.open-broker.ru/economy/indeks-lernera/
• https://www.cjournal.cz/files/91.pdf