1. Average life calculations are intended to measure the weighted average time until receipt of principal payments. Some measures of expected life may include the accrued interest as a cash-flow. These increases in the principal balance can enter the calculation as negative weights applied to the elapsed time to early payment dates, and the actual principal payments as positive weights applied to the elapsed time to later payment dates. By placing negative weights on small numbers and positive weights on large numbers, the results can be larger than the remaining term of the underlying collateral (for example, a number of years greater than 30). To avoid this unrealistic result, the convention in the CMO market is to exclude from the calculation all increases in the factor or balance, with the understanding that this method can substantially understate the true interest rate sensitivity of a security.