Aggregate selected monthly owner costs are calculated by adding together all the selected monthly owner costs for occupied housing units in an area. Aggregate selected monthly owner costs are subject to rounding, which means that all cells in a matrix are rounded to the nearest hundred dollars. (For more information, see "Aggregate" under "Derived Measures".)

The data on mobile home costs were obtained from answers to long-form questionnaire Items 53a and 53b, which were asked on a sample basis at owner-occupied mobile homes. Questionnaire Item 53a asks if there is an installment loan or contract on the mobile home. This is a payment plan for mobile homes similar to buying a car or appliance. If the mobile home is not permanently attached to the land it may not be considered real estate and thus will not have a mortgage. With an installment loan, the buyer pays a specified amount per month for a specified number of months. The mobile home is the collateral for the loan, similar to a car loan.

The data derived from Question 53b include the total annual costs for installment loan payments, personal property taxes, land or site rent, registration fees, and license fees on owner-occupied mobile homes. The instructions are to exclude real estate taxes already reported in long-form questionnaire Item 49 or personal property taxes in arrears from previous years.

Costs are estimated as closely as possible when exact costs are not known. Amounts are the total for an entire 12-month billing period, even if they are paid by someone outside the household or remain unpaid.

The data from this item are added to payments for mortgages; real estate taxes; fire, hazard, and flood insurance payments; utilities; and fuels to derive selected monthly owner costs for mobile home owners.

The data on mortgage status were obtained from answers to long-form questionnaire Items 47a and 48a, which were asked on a sample basis at owner-occupied housing units. "Mortgage" refers to all forms of debt where the property is pledged as security for repayment of the debt, including deeds of trust; trust deeds; contracts to purchase; land contracts; junior mortgages; and home equity loans.

A mortgage is considered a first mortgage if it has prior claim over any other mortgage or if it is the only mortgage on the property. All other mortgages, (second, third, etc.) are considered junior mortgages. A home equity loan is generally a junior mortgage. If no first mortgage is reported, but a junior mortgage or home equity loan is reported, then the loan is considered a first mortgage.

In most census data products, the tabulations for "Selected Monthly Owner Costs" and "Selected Monthly Owner Costs as a Percentage of Household Income in 1999" usually are shown separately for units "with a mortgage" and for units "not mortgaged." The category "not mortgaged" is comprised of housing units owned free and clear of debt.