The yield on an issue of bonds is used to apply investment yield restrictions under section 148(a) and to compute rebate liability under section 148(f). Yield is computed under the economic accrual method using any consistently applied compounding interval of not more than one year. A short first compounding interval and a short last compounding interval may be used. Yield is expressed as an annual percentage rate that is calculated to at least four decimal places (e.g., 5.2525 percent). Other reasonable, standard financial conventions, such as the 30 days per month360 days per year convention, may be used in computing yield but must be consistently applied. The yield on an issue that would be a purpose investment (absent section 148(b)(3)(A)) is equal to the yield on the conduit financing issue that financed that purpose investment. The Commissioner may permit issuers of qualified mortgage bonds or qualified student loan bonds to use a single yield for two or more issues.