The following summarizes the movement in the provision for doubtful accounts receivable:

There is no concentration of credit risk with respect to trade receivables, as the Group has a large number of internationally dispersed customers.

The Group applies the IFRS 9 simplified approach to measuring expected credit losses, which uses a lifetime expected loss allowance for all trade receivables within stage 1 and 2. The estimated expected loss rates are based on historical credit losses and are adjusted to reflect current and forward-looking information on macroeconomic factors affecting the ability of the customers to settle the receivables. The Group recognizes impairment of trade receivables in »Selling, general and administrative costs« in the income statement.

The maximum credit risk on trade receivables is equal to their fair value.

Collaterals are only required in rare cases (2019: CHF 2 million, 2018: CHF 2 million).

The loss allowance for trade receivables as at 31 December 2019 and 2018 was determined as follows:

The carrying amounts of the Group’s trade receivables are denominated in the following currencies:

As of 31 December 2019, »Total trade receivables – net« include an amount of CHF 110 million (2018: CHF 145 million) past due, but not impaired. These relate to a number of customers for whom there is no recent history of default.