The opportunity cost of college also increased due to the higher wages of high school graduates. Over the past decade, the cost of in-state tuition for a 4-year education increased by one-third, with tuition inflation rates decreasing in the recent decade. A 2014 study by economists Jaison Abel and Richard Deitz found that the opportunity cost of attending college amounts to $120,000 due to forgone wages, with the total cost of college amounting to an estimated $150,000 when also factoring in out-of-pocket expenses. Returns on investment Human capital in the form of education shares many characteristics with physical capital. Both require an investment to create and, once created, both have economic value. Physical capital earns a return because people are willing to pay to use a piece of physical capital in work as it allows them to produce more output. To measure the productive value of physical capital, we can simply measure how much of a return it commands in the market. In the case of human capital calculating returns is more complicated – after all, we cannot separate education from the person to see how much it rents for. To get around this problem, the returns to human capital are generally inferred from differences in wages among people with different levels of education. Hall and Jones have calculated from international data that on average the returns on education are 13.4% per year for first four years of schooling (grades 1–4), 10.1% per year for the next four years (grades 5–8) and 6.8% for each year beyond eight years. Thus someone with 12 years of schooling can be expected to earn, on average, 1.1344 × 1.1014 × 1.0684 = 3.161 times as much as someone with no schooling at all. Higher levels of educational attainment can increase lifetime earnings, impacting the return on investment (ROI) of education. In the US at the college and university level, each level of degree attainment significantly increases lifetime earnings as more education is achieved. Lifetime ROI is significantly higher at lower levels of educational attainment than at higher levels (1,200.8% for an Associate's degree vs. 287.7% for a Bachelor's degree). While higher levels of degree attainment can increase lifetime earnings, the ROI decreases at the doctoral level compared to a master's degree. In higher education, ROI also varies significantly depending on the degree concentration. Degree concentration matters when examining the ROI of Bachelor's degrees,