The following table summarizes the changes in allowance for doubtful debts for each of the years in the three-year period ended December 31, 2017:
| Year ended December 31, | ||||||||||||
| 2015 | 2016 | 2017 | ||||||||||
| RMB | RMB | RMB | ||||||||||
| (restated) | ||||||||||||
|
At beginning of year |
2,478 | 2,935 | 3,402 | |||||||||
|
Impairment losses for doubtful debts |
2,172 | 2,203 | 1,962 | |||||||||
|
Accounts receivable written off |
(1,715 | ) | (1,736 | ) | (1,522 | ) | ||||||
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At end of year |
2,935 | 3,402 | 3,842 | |||||||||
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