[ D] Earnings are an indicator of the value of skills on the labour market. Therefore, it is useful to look at the level as well as at the distribution of earnings in respect to education. According to the human capital theory, people invest in education in order to enhance their future earnings. Thus, one can estimate a so-called "return" to educational investment, that takes into account the costs and the benefits of education. Doing so, one can assess the labour market value of the various types and levels of education.
The questions that are typically examined in this field are: