In the past decades, public private partnership (PPP) has been widely used as an effective approach to stimulating private investment and improving supplies for public services. The optimization of the concession period is very critical for the successful implementation of PPP projects. This paper takes decision making activities of the concession period as a bargaining game process between public and private sectors, and presents a mathematical model to calculate the optimal concession period under a series of assumptions. Backward induction is applied to solve this optimization problem. Finally, numerical simulation is used to further verify the model and analyze the influence of different factors on the optimal concession period. The results indicate that the optimal toll rate should be determined based on the average level of service provided. Factors such as the risk interest rate, initial investment, reservation utility are positively correlated with the optimal concession period, while the private equity ratio and the toll rate are negatively correlated with it. The current study is a useful supplement to previous research works as it incorporates the influence of public equity and time value of money into the construction of model. Moreover, it can act as a helpful reference for both public and private sectors in negotiations concerning related issues.

In recent years, there is a substantial debate on the wage differences in China. Yet most empirical studies of wage gap concentrate only on short-run changes due to the lack of high-quality data. In this paper, we study the wage gap between urban and rural labor in China using the Chinese Health and Nutrition Survey (CHNS) data spanning several years. By the methods of OLS regression and Oaxaca decomposition we have two main findings: first, the wage gap resulting from household register discrimination have been gradually decreasing since the Lewis transition, and the growth rate of migrant workers' wages obviously exceeded the accumulation rate of their human capital in 2011; second, the wage disadvantage of the rural labor mainly comes from the weakness of its human capital, compared with that of the urban labor. These findings would remind us of the rising welfare inequality problem in China against the background that the accumulation of human capital on rural labor have slowed down after the Lewis transition. In the end, we come up with several suggestions for policy reform. First of all, the central government should distribute more educational resources into rural areas specifically. In addition, the local government and enterprises should increase specific investment in skills training for migrant workers. Last but not least, the governments ought to gradually abolish the related rules that restrain migrant workers from obtaining urban public services, which will help to equalize the social welfare in China eventually.