Allowances for Receivables and Sales Returns
The allowances for receivables were as follows (in millions, except percentages):
(1) Calculated as allowance for credit loss on lease receivables as a percentage of gross lease receivables and residual value before unearned income.
The allowance for doubtful accounts is based on our assessment of the collectibility of customer accounts. We regularly review the adequacy of these allowances by considering internal factors such as historical experience, credit quality and age of the receivable balances as well as external factors such as economic conditions that may affect a customer’s ability to pay as well as historical and expected default frequency rates, which are published by major third-party credit-rating agencies and are updated on a quarterly basis. We also consider the concentration of receivables outstanding with a particular customer in assessing the adequacy of our allowances for doubtful accounts. If a major customer’s creditworthiness deteriorates, if actual defaults are higher than our historical experience, or if other circumstances arise, our estimates of the recoverability of amounts due to us could be overstated, and additional allowances could be required, which could have an adverse impact on our operating results.
The allowance for credit loss on financing receivables is also based on the assessment of collectibility of customer accounts. We regularly review the adequacy of the credit allowances determined either on an individual or a collective basis. When evaluating the financing receivables on an individual basis, we consider historical experience, credit quality and age of receivable balances, and economic conditions that may affect a customer’s ability to pay. When evaluating financing receivables on a collective basis, we use expected default frequency rates published by a major third-party credit-rating agency as well as our own historical loss rate in the event of default, while also systematically giving effect to economic conditions, concentration of risk and correlation. Determining expected default frequency rates and loss factors associated with internal credit risk ratings, as well as assessing factors such as economic conditions, concentration of risk, and correlation, are complex and subjective. Our ongoing consideration of all these factors could result in an increase in our allowance for credit loss in the future, which could adversely affect our operating results. Both accounts receivable and financing receivables are charged off at the point when they are considered uncollectible.
A reserve for future sales returns is established based on historical trends in product return rates. The reserve for future sales returns as of July 27, 2019 and July 28, 2018 was $84 million and $123 million, respectively, and was recorded as a reduction of our accounts receivable and revenue. If the actual future returns were to deviate from the historical data on which the reserve had been established, our revenue could be adversely affected.
Table:
 | July 27, 2019 | July 28, 2018
Allowance for doubtful accounts | $136 | $129
Percentage of gross accounts receivable | 2.4% | 2.3%
Allowance for credit loss—lease receivables . | $46 | $135
Percentage of gross lease receivables (1) | 1.8% | 4.7%
Allowance for credit loss—loan receivables | $71 | $60
Percentage of gross loan receivables | 1.3% | 1.2%


Question: What is the allowance for doubtful accounts based on?
Answer: our assessment of the collectibility of customer accounts