For instance, there is evidence that pay is important to an internship program's success, particularly when the program is a recruiting program. In a recent survey by the National Association of Colleges and Employers, nearly all responding organizations who use their internship programs as part of their college recruiting effort pay their interns.

In turn, pay is tied to interns' satisfaction with the internship program. NACE's study of the students from the Class of 2007 found that students who reported dissatisfaction with their internships tended to be in unpaid programs. 

The NACE survey found that a variety of factors affect internship salaries, including the student's degree level, year in school, and field of study, as well as the organization's industry and location. For example, a senior studying for a bachelor's in one of the engineering fields averages $19.44 per hour, while a sophomore engineering student averages $16.65.

Overall, employers reported offering their undergraduate interns an average of $16.33 per hour -- and nearly $25 per hour for interns at the master's degree level.

The purpose of the program also affects salary.

"Average hourly salaries to interns are even higher among those organizations that use their internship programs to help them recruit full-time hires," said Marilyn Mackes, NACE executive director.

Among respondents whose organizations have recruiting-focused programs, the average hourly salary to an undergraduate intern is $16.69, and $25.93 to a master's level intern.

"The paid internship is an increasingly important tool for organizations that want to hire new college graduates," said Mackes.