The average worker with a bachelor’s degree can expect to make about $1 million more over the course of his or her lifetime than someone with only a high school diploma. The average tuition at a private university now costs $30,094 per year or $120,376 for four years of education, making college appear to be a good investment.

Yet 37% of those with only a high school education make more than the median earnings of workers with a bachelor’s degree. The bottom line is that not all colleges are equal, and some generate better returns on a tuition investment than others.

Related: Do parents 'owe' their children a college education?

This time of year, high school seniors across the country have to decide which of the 4,450 higher-education institutions to dedicate their money and time to. In order to facilitate the tough decision-making process, Matt Schifrin, managing editor at Forbes, created the Grateful Grads Index. The index calculates the return on investment (ROI) of private, degree-granting colleges by looking at both the percentage of alumni who end up donating and how much they donate.

“Parents and students want to know how to measure the value of a college,” says Schifrin. For alumni, "if you’re successful and you’re happy [with your college education] then you’re going to give money back, so we looked at the median private donations over 10 years … we also factored in alumni participation, in other words what percentage of alums donate each year.”

Related: Only 150 of 3500 U.S. Colleges Are Worth the Investment: Former Secretary of Education

Most of the colleges on Schifrin’s list are the ones you would expect to be there. Princeton clocks in at number one and Dartmouth, Brown and Stanford are all in the top 10. There are however, some surprise names in the top 10, like Davidson College — a small liberal arts school in North Carolina that has produced 23 Rhodes Scholars in the past decade.

Other surprises involved some of the top schools in the country. “Harvard came in number 20 on our list and that’s mostly because while they have a very high dollar amount of donations, [but] only 19% of their alums actually donate," says Schifrin. "At Columbia University, an amazing school, only 13% of alums donate."

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