SEATTLE — A new study says fewer than 55 percent of four-year college students graduate with a degree within six years.

The report by conservative think-tank American Enterprise Institute was partially paid for by the Bill & Melinda Gates Foundation.

Researchers found college completion rates vary widely. But in general, graduation rates are better at more competitive schools. The researchers say those gaps are mostly due to the different types of students the more selective schools enroll.

The study also compares graduation rates within similar kinds of schools and found some differences. For example, San Diego State graduates 56 percent of its students within six years and California State University in Los Angeles has a 31 percent graduation rate.

Using the U.S. Department of Education statistics, the report compares about 1,300 colleges and universities and points out the best and worst graduation rates.

Transfers to other universities are lumped in as part of the dropout rate, the researchers point out, so the picture at some universities is not as bad as the report concludes. The report argues that transfers make up only a small part of the difference between schools with the best and worst graduation rates within a given category.

The researchers argue that parents and students should have access to this information to help them choose a college, considering thousands of tuition dollars are involved.

“We believe that the graduation rate measure included here should be just the beginning of a deeper inquiry into college success,” they wrote.

The information in the report would be a useful tool for parents and students comparing two similar institutions, the researchers said.

For example, Heritage University and Walla Walla University, two small private colleges in Washington state, have very different graduation rates. Walla Walla graduates 53 percent of its students within six years and Heritage graduates 17 percent.

At every level of competitiveness, most of the colleges with the best graduation rates are private schools.

Some exceptions include Kansas State University in Manhattan, Kan., the University of Wisconsin at Platteville, Shippensburg University of Pennsylvania, the University of Texas Health Science Center at San Antonio, the University of Pittsburgh at Johnstown and James Madison University in Harrisonburg, Va.

At the top of the list of colleges with moderately competitive admission policies — median freshman SAT scores of between 500 and 572 and minimum high school grades ranging from C to B-minus — is the College of Our Lady of the Elms of Chicopee, Mass., followed by Yeshiva University in New York City with graduation rates of 89 percent and 84 percent respectively.

On the bottom of that list are Texas Southern University of Houston, Texas, and Voorhees College of Denmark, S.C., both with a 12 percent graduation rate.