Krajcik and Blumenfeld, in their landmark paper "Project-Based Learning" (2006) outline five necessary elements that a unit needs to be considered Project Based Instruction (PBI). These are:

1. The presence of a driving question.

2. Students engage in authentic, situated inquiry.

3. There is a collaboration of students among students and students with the instructor.

4. Students are scaffolded with learning technology.

5. There is a tangible product that directly addresses the driving question.

I believe that if these qualities are met, projects can be a highly effective method of learning -- it gives a pressing need for information that should drive students to dive into the relevant knowledge.