How can economist precisely compare standards of living in different countries?

Economist precisely compare standards of living in different countries by the GDP of the countries. The gross domestic product (GDP) is one of the primary indicators used to gauge the health of a country's economy. It represents the total dollar value of all goods and services produced over a specific time period; you can think of it as the size of the economy.

money spent each year, many things like that, but mostly the population