Thesis: This thesis is composed of six essays on youth and the labour market on the one hand, and on the use of subjective variables in health economics on the other hand. In most developed countries, young people often face great and increasing difficulties when entering the labor market, especially since the Great Recession. The first two chapters analyse French programs aimed at helping youth facing these difficulties.We evaluate the impact of extending the minimum income to young people (Chapter 1) and of strengthening the main training program for youth (Chapter 2). The third and fourth chapters focus on the unemployment level when young people enter the labor market, analyzing its impact on their educational attainment (Chapter 3), and on their wage and employment prospects throughout their career (Chapter 4). Moreover, the increasing use of subjective variables by economists raises the question of their reliability and robustness. In the last two chapters, we explore how the formulation of subjective questions shape the collected answers, for two variables often used in health economics : foregone health care due to cost (Chapter5) and subjective health (Chapter6). Using the split sample method, we compare the responses of several random samples of respondents, which answer to different formulations of the same question.