the definition of a market, therefore meaning you cannot use it to cross-examine different industries, or do analysis over time as the industry changes. Lerner index The Lerner index is a widely accepted and applied method of estimating market power in a monopoly. It compares a firm's price of output with its associated marginal cost where marginal cost pricing is the "socially optimal level" achieved in market with perfect competition. Lerner (1934) believes that market power is the monopoly manufacturers' ability to raise prices above their marginal cost. This notion can be expressed by using the formula: Where P represents the price of the good set by the firm and MC representing the firm's marginal cost.The formula focuses on the nature of monopoly and emphasising welfare economic implications of the Pareto optimal principle. Although Lerner is usually credited for the price/cost margin index, the generalized version was fully derived prior to WWII by Italian neoclassical economist, Luigi Amaroso. Connection with Competition Law Market power within competition law can be used to determine whether or not a firm has unfairly manipulated the market in their favour, or to the detriment of entrants. The Sherman Antitrust Act of 1890 under section 2 restricts firms from engaging in anticompetitive conduct by utilising an individual firm’s power to manipulate the market or partake in anticompetitive acts. A firm can be found in breach of the act if they have leveraged their market power to unfairly gain further market power in a manner that is detrimental to the market and consumers. The measurement of market power is key in determining a breach of the act and can be determined from multiple measurements as discussed in measurements of market power above. In Australia, consumer law allows for firms to have significant market power and utilise it, as long as it is determined to not have “the purpose, effect or likely effect of substantially lessening competition” Elasticity of demand The degree to which a firm can raise its price above marginal cost depends on the shape of the demand curve at a firm's profit maximising level of output. Consequently, the relationship between market power and the price elasticity of demand (PED) can be summarised by the equation: The ratio P/MC is always greater than 1 and the higher the P/MC ratio, the more market power the firm possesses. As PED increases in magnitude, the P/MC ratio approaches