Sharon This measure is a target as defined by HUD and applies to homeownership and rental at all income levels. If housing costs exceed this level it is defined and a cost burden adjusted for family size. So in theory as the combined housing cost fluctuates and/or income (up or down) the ratio of cost to income and affordability index will change (increased/decreased). This is not to be confused with mortgage lender underwriting standards (debt to income ratios) which is a different calculation.
Beth Ratio of housing value to HH income increased from 1.9 to 4.2 over 20 year period