Table:
|  | 2019 | 2018 |
| --- | --- | --- |
| Unbilled | $ 22,827 | $ 24,282 |
| Billed: |  |  |
| Current | 1,286 | 1,465 |
| Past due | 236 | 271 |
| Device payment plan agreement receivables, gross | $ 24,349 | $ 26,018 |
Paragraphs:
Subsequent to origination, the delinquency and write-off experience is monitored as key credit quality indicators for the portfolio of device payment plan agreement receivables and fixed-term service plans. The extent of collection efforts with respect to a particular customer are based on the results of proprietary custom empirically derived internal behavioral-scoring models that analyze the customer’s past performance to predict the likelihood of the customer falling further delinquent.
These customer-scoring models assess a number of variables, including origination characteristics, customer account history and payment patterns. Based on the score derived from these models, accounts are grouped by risk category to determine the collection strategy to be applied to such accounts.
Collection performance results and the credit quality of device payment plan agreement receivables are continuously monitored based on a variety of metrics, including aging. An account is considered to be delinquent and in default status if there are unpaid charges remaining on the account on the day after the bill’s due date.
At December 31, 2019 and 2018, the balance and aging of the device payment plan agreement receivables on a gross basis was as follows:
Question:
How is the delinquency and write-off experience monitored?
Answer: as key credit quality indicators for the portfolio of device payment plan agreement receivables and fixed-term service plans.