As all accountants know, there is no such thing as an objective and definitive set of numbers - it all depends on how you arrange them, and what work you want them to do.

Each year the FM Top Companies survey provokes objections from companies that say some of the figures that are used in compiling our tables do not tally with their own. However, it has always been the case that some of the figures used in this publication differ from those published by the companies. All the published financial statements are standardised because the accounting conventions used by companies differ, making it impossible to compare and rank companies using only their own published data. Those interested in how the Bureau for Financial Analysis has calculated its results are referred to "Definitions".

The other difficulty is that companies have different year-ends, which means that a company with a March 2008 year-end will obviously be represented by more recent numbers than a company with a June 2007 year-end. We have considered trying to freeze all companies' financial performance at a particular date, but it soon became clear that the quality of information would suffer. Even in an era of real-time information - in order for companies to be comparable - there is no way past using their annual results. Occasionally this leads to apparently contradictory conclusions, as two companies in the same sector may appear to be reacting differently to environmental developments.

However, the value lies in the fact that each company is judged on the same period as in the previous edition, so there is an internal consistency. It's not a perfect snapshot of the listed corporate sector, but it's the best we've got.