Comparability problems pre IFRS included:

Leases There was no distinction between operating and finance leases for many
European countries.

Pensions Some companies adopted IFRS several years before they had to by
law in 2005. IFRS caused some big one-off moves to operating assets as pension
deficits came on to the balance sheet. British Airways, for example, had a pension
deficit of £1.3bn. Suddenly recognising this on its balance sheet generated a
one-off benefit to operating assets.

Consolidation Certain GAAP (Generally Accepted Accounting Principles) regimes
in the past allowed some companies not to consolidate subsidiaries. Again this meant
that it was almost impossible to compare operating assets and cash flow on a fair basis.

Where is the cash flow? Prior to IFRS, Spanish companies were not obliged to
produce cash flow statements, and many of them didn’t.

Goodwill Countries differed in their treatment of goodwill. In the UK, most companies
prior to 2000 just wrote off to reserves (reducing operating assets unfairly).
In Europe most companies amortised goodwill, again making comparisons difficult.

The above considerations suggest that older data is possibly unreliable. But
The Cashflow Solution tested well even when polluted data was used, and similar
approaches have tested better in the US, where data is cleaner.