Some notes on applying the Herfindahl--Hirschman Index

The Herfindahl--Hirschman Index is one of the most commonly used indicators to detect anticompetitive behaviour in industries. In fact, an increase in the value of the index is usually interpreted as an indicator of actions which may lessen competition or even create a monopoly. In this article we show that this is not always the case, since in certain instances it is impossible to detect cooperation. We also show an example when competition even has a decreasing effect on the value of the index.