Most local governments in the United States impose a property tax as a principal source of revenue. This tax may be imposed on real estate or personal property. The tax is nearly always computed as the fair market value of the property times an assessment ratio times a tax rate. Values are determined by local officials, and may be disputed by property owners. One advantage to the State, of the property tax over the sales tax or income tax is that the revenue always equals the tax levy, unlike the other taxes. The property tax always produces the required revenue for municipalities' tax levies. A disadvantage to the taxpayer is that their tax liability is fixed, while their income is not.