Consolidated Financial Statements Notes 1. Principles 
The major debtors or debtor groups of trade accounts receivable were US Medicare and Medicaid health care programs with 13% as well as private insurers in the US with 17% at December 31, 2007. Other than that, the Fresenius Group has no significant risk concentration, due to its international and heterogenous customer structure.
The allowance for doubtful accounts was € 223 million and € 218 million as of December 31, 2007 and December 31, 2006, respectively.
Sales are invoiced at amounts estimated to be receivable under reimbursement arrangements with third party payors. Estimates for the allowance for doubtful accounts are mainly based on historic collection experience, taking into account the aging of accounts receivable and the contract partners. The Fresenius Group believes that these analyses result in a well-founded estimate of allowances for doubtful accounts. From time to time, the Fresenius Group reviews changes in collection experience to ensure the appropriateness of the allowances.
Deterioration in the ageing of receivables and collection difficulties could require that Fresenius Group increases the estimates of allowances for doubtful accounts. Additional expenses for uncollectible receivables could have a significant negative impact on future operating results.
d) Self-insurance programs
Under the insurance programs for professional, product and general liability, auto liability and worker’s compensation claims, the largest subsidiary of the Fresenius Group, located in North America, is partially self-insured for professional liability claims. For further details regarding the accounting policies for self-insurance programs, please see Note 1.IV.y, Summary of significant accounting policies.
