One simple way to measure this that’s widely used by Governments and expert bodies such as the OECD is the number of people living below a poverty line. This report uses two poverty lines, 50% of median household income (half of the ‘middle’ income for all households) and 60% of median income.
The lower of these poverty lines (which is used by the OECD) equates to a very austere living standard: a disposable income of less than $358 per week for a single adult (higher for larger households to take account of their greater costs). This research also takes account of people’s housing costs in measuring poverty.