Washington property tax laws require the Department of Revenue to annually determine the level of assessment in each county in a process described as the Ratio Study. The level of assessment in a county means how statistically close the assessor is to 100% of true and fair value.  To conduct this study, the Department must audit at least one-half of one percent of all personal property listings each year.

The county ratio is calculated for both real and personal property and is used by the Department of Revenue to ensure that the state school levy is applied fairly throughout the state. Owners of like property should pay the same amount of tax, based on market value, no matter where the property is located.

In order to perform the Ratio Study, property accounts are randomly selected for audit, which is then compared to the county's assessed value.