There should be a universal human standard to define what extreme poverty is--i.e. the amount needed to secure food, shelter, and clothing--and then that amount should be assessed country-by-country (or region-by-region) by an independent body. The number of $3 per day is well above the "basic needs" threshold in some of the poorest countries, and well below it in the US, for example.
Makes you wonder what the real purpose of that number was. Must have served some agenda, because saying some people live on less than $3 (when it's not a fair statement) definitely could serve a purpose.
Does the person buy food and basics per day? Then don’t worry about what the dollar amount equates to. It’s a ridiculous metric when it comes to measuring abject poverty.
This is addressed in the article - see the section titled "Estimating comparable national distributions". (In short: income is being scaled relative to purchasing power parity.)
