Amid the booming equity markets of the 1990s, listed companies looked poised to triumph over all others, write Bill Meehan and Richard Dobbs of McKinsey & Company. Family companies, employee and state-owned corporations as well as mutuals and partnerships headed for stock markets in a stampede of initial public offerings.

Looking across the business landscape today, however, the heterogeneity of ownership remains striking. The success of private equity has challenged the notion that the discipline imposed on managers by public markets is the path to optimal business performance. Higher energy prices have strengthened the economic position of state-owned oil companies. The growing economic importance of Asia, with its rich tradition of family business, has reinforced the role both of private companies and of quoted groups with very influential family investors.

Such diversity poses questions important to investors, managers and government ministers alike. What are the strengths and weaknesses of different forms of corporate ownership? To what extent can management practices be transferred from one to another? Under what circumstances could it make sense to change ownership structure - from, say, public to private - in pursuit of improved productivity or other advantages? In a world of competing ownership models, these questions are compelling.

The FT Non-Public 150 for the first time brings together the world's biggest unquoted companies in a single list. Based on research by McKinsey, its aim is simple: to stimulate informed debate about the impact of different ownership structures on corporate performance and health.

In compiling the list, we looked for companies that passed three main tests. First, their shares were not listed on a public stock market. Second, they were neither a joint venture nor a subsidiary of a larger entity. Third, they delivered goods or services into a commercial marketplace.

This third test explains why armies, government health systems, religious organisations and charitable foundations did not make the cut, in spite of their sometimes considerable economic muscle.

Note, however, that the FT Non-Public 150 is not a performance ranking. Judging the relative value creation of the companies on the list would require far more information than is available in the public domain for the majority of them.

For the same reason, the valuations according to which the list is ordered are not pin-point accurate. The figures represent a best guess - based solely on often incomplete public information - at the market value of these entities had the shares been listed on public markets at the end of last year.

This first iteration of the FT Non-Public 150 is without doubt a work in progress. Our hope is that the insight of Financial Times readers will enable us to refine both the list itself and our collective grasp of the strengths and weaknesses of different forms of ownership in managing corporate performance and health.