The Department of Education recently released their overview of a number of quantitative studies that have attempted to glean the impact of blended and online instruction on student learning.
For time-strapped readers, Inside Higher Ed (June 29, 2009) offers the following summary of the report:
The study found that students who took all or part of their instruction online performed better, on average, than those taking the same course through face-to-face instruction. Further, those who took "blended" courses -- those that combine elements of online learning and face-to-face instruction -- appeared to do best of all. That finding could be significant as many colleges report that blended instruction is among the fastest-growing types of enrollment.