About what proportion of the variation of wages across workers is explained by factors that can be measured? What are the other factors that explain wage differences but are difficult to measure?

Explanation:

Easy Measuring of the wages could be from data collected such as from the International Labour Organization. They compare wages from state-to-state, country-to-country and from men-to-women in the workplace.

Hard factors explaining wage differences come from classical economic theory (standard) and labour market imperfections due to varying circumstances from country to country. This is largely due to the differing economic standards and regulations that are ran by different countries