Problem-based learning (PBL), although increasingly popular, is expensive and labor-intensive to implement. Research evidence has not yet established that PBL's benefits are worth its cost.

Studies of PBL conducted in naturalistic educational environments are typically so loosely controlled as to make interpretation problematic. Problems with these studies include self-selection to conditions, uncontrolled variation across conditions, and multidimensional treatment conditions.

Two tightly controlled experiments were conducted in a college classroom setting. In both, a crossed design, with randomized assignment and control over potential confounds, was employed. Two equivalent college classes of a science course taught by the same instructor were compared in their learning of two concepts of equivalent difficulty over a period of four weeks. In Study 1, class A learned the first concept via lecture/discussion and the second concept via PBL. Class B learned the first concept via PBL and the second via lecture/discussion. Students' performance was assessed 4 times, immediately after each concept was introduced (at weeks two and four) and again at weeks 9, 13 and 16. Results indicated students' superior mastery of the concept learned via PBL in terms of understanding, integration and application.

Study 2 examined the sources of PBL's effectiveness, specifically whether the characteristic feature of social collaboration is a necessary component of PBL. This was accomplished using the same design, procedures and instruments employed in Study 1 but comparing a group engaged in individual PBL experience vs. a group engaged in the more typical small-group PBL format. Results indicated that collaboration is not an essential component of PBL, as revealed by a lack of a significant difference in students' performance across these two conditions. Performance in both conditions remained superior to that in the Lecture/discussion condition.

The results of this research have the potential to be drawn on as a basis for decision-making in educational policy and practice, achieving the desirable and currently much-discussed goal of basing such decisions on rigorous empirical evidence.