看来你和你引文的作者一样，没有把household和family的区别搞清楚。

the median household income of $50,054 and divide it up over 12 months, it comes to about $4000 a month.

About half of all American households are making more than that and about half of all American households are making less than that.

So can an average family of four people make it on just $4000 a month?

Well, first of all you have got to take out taxes. After accounting for all forms of taxation you will be lucky if you have $3000 remaining.

上面是你的引文：

在统计收入时：

Family，即所谓的Census Family，要一个大人和至少另外一个人（大人和小孩）的才算。

统计Household则是按照住址来统计的，一个单身刚工作的算一个household。但是统计family收入时，是不包括只有一个人的情况的，也就是排除在统计样本外。

http://www.justice.gov/ust/eo/bapcpa/20110315/bci_data/median_income_table.htm

上面统计出来的Family Income，4口之家，最穷的州都有$53,709，比你那位作者的$50,054高多了。