How to measure the market power of banks in the lending business accurately: a segment-based adjustment of the Lerner Index

Numerous empirical studies use the price-cost-margin-based Lerner Index (LI) to assess the general market power of banks. A common procedure within those LI applications is to approximate the market price required for the LI measurement as the ratio of a bank's total revenues to total assets. We discuss the major flaws of this aggregated procedure and propose an adjusted (i.e. business segment-orientated) LI approach, which is then applied to assess the market power of banks in the specific lending business at the country level. Our empirical study is based on an original data set containing all interest-related categories (weighted by the respective loan as well as deposit volumes) in the countries of the European Monetary Union zone (EMU) from 2003 to 2013. Our results reveal that the country-specific market power of banks in the lending business has been substantially underestimated in previous studies based on aggregated outputs. For example, averaged across the five most important economies in the EMU, we detect a calibration factor of four. Our findings corroborate the economic notion that the interest-bearing lending business is a more locally separated, and thus profitable, segment in which competition is attenuated.