The US Census Bureau defines lower class as the lowest 20% of incomes.

Pew defines it as the lowest 2/3 of the median.

If the highest income is $100, 20% is $20.

At 2/3 the median, if the median is $30, lower-class is $20.

Is there a reason they’re tying income classes to people within those classes, and not what it costs to survive within the economy?