This paper presents a harmonized data set over the period 1972-2002, containing two-yearly data on the number of non-agricultural business owners and the size of the labour force for 23 OECD countries, as well as the quotient of these two variables which is called the business ownership rate of a country. The data set is called COMPENDIA, which means COMParative ENtrepreneurship Data for International Analysis. It has been constructed by EIM Business and Policy Research, using OECD statistics (in particular OECD Labour Force Statistics) as well as other relevant sources. We make an attempt to make business ownership rates comparable across countries and over time. This is not straightforward as different countries measure business ownership or self-employment in different ways. The definition used in COMPENDIA includes both the unincorporated and the incorporated self-employed. This paper describes the methodology used for constructing the COMPENDIA data base. It also presents the business ownership rates for the 23 countries. Special attention is paid to the United States. This country alone accounts for about 30% of all self-employed reported in the COMPENDIA data set.