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

Of the total provision for doubtful accounts receivable, the following amounts concern trade receivables that were individually impaired:

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 recognizes the 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 (2017: CHF 2 million, 2016: CHF 2 million).

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

As of 31 December 2017, »Total trade receivables – net« include an amount of CHF 141 million (2016: CHF 143 million) past due, but not impaired. These relate to a number of customers for whom there is no recent history of default.

The ageing analysis of these trade receivables is as follows: