The number of total building permits in cities and building permits for single-family homes in cities come from the Building Permits Survey conducted by the U.S. Census Bureau.
Economists use index numbers for quick comparisons between sets of data across time. The Building Permits in Cities, Single Family (Indexed to 2005) measure is calculated by identifying 2005 as the "base year" and scaling all of the other Building Permits, Single Family results off of that year.
For example, the Building Permits in Cities, Single Family value for Chicago in 2005 is represented by 1.0 (obtained by dividing the value by itself, 37470/37470= 1.0) and the numbers from other years are similarly scaled down. The Building Permits in Cities, Single Family (Indexed to 2005) value for 2018 is 0.23 obtained by dividing 2019 Building Permits, Single Family value by the base year’s Building Permits, Single Family value ( 8416/37470 = 0.2246)