1. A vacancy rate becomes a significant metric to measure when it is in a range from to 3 percent.

2. Unlike some other real estate metrics, like sales volume, rent growth, and median home prices, vacancy rates are calculated by the national data-gathering organization and not by the individual real estate agent. The vacancy rate is nationally averaged and calculated as a percentage of the entire housing stock that is looked at.

To illustrate the industry-wide weighting of the data, the latest data on vacancy rates for the U.S. housing stock showed a vacancy rate nationwide in December 2016 of 2.4 percent, with the national maximum vacancy rate of 3 percent and national minimum vacancy rate of 1.2 percent.

3. There are four occupancy categories studied in vacancy rates: six or more units in a building, five or six units in a building, four or five units in a building, and one to three units in a building.

4. Vacancy rate data is approximated as a percentage by dividing the Housing Occupied Housing Unoccupied from the Point Trend of Housing Units.