Inputs to teacher productivity

Some strong negative results:

As with most previous research, we found no relationship between a teacher’s earning a master’s degree, certification, or years of experience and the teacher’s classroom performance as measured by student test scores. Though we found that some pedagogy course work was related to teaching effectiveness, the magnitude of the effect was mild: even very detailed information about the teacher’s preparation in college told us very little about how effective that teacher would be in the classroom.

If you want to teach, say, geometry to high-school students, it surely helps to know geometry. At my university, you can take a course with this description:

Selected topics from the foundations of Euclidean and non-Euclidean geometries. Includes the study of spherical and hyperbolic geometries, as well as transformational geometry, with techniques from linear algebra. Intended primarily for students seeking secondary certification.

After that course, your understanding of geometry will go much deeper than it did in high school. However much or however little your future geometry students will benefit from your deeper understanding, my point is that this is an undergraduate course. From another undergraduate course description:

Selected topics from secondary school mathematics. Content, materials, and contemporary issues specific to teaching of mathematics at the secondary school level.

That course has a corequisite; from its description:

This course will focus on field-based supervision of elementary and secondary education pre-service teachers.

After taking such courses, why would I be surprised to learn that you won't help your students by getting a master's degree? There are diminishing returns to education. For an example from higher education, graduate students who just got their bachelor's degrees are assigned to teach lower-level undergraduate courses. As a math PhD student, I taught a trigonometry class and was a TA for calculus classes and for a linear-algebra-plus-differential-equations class. There were no graduate course prerequisites for these teaching assignments. My graduate courses were intended to make me a better scholar, not a better educator.

More surprising is the failure to detect a student test score benefit from more experienced teachers. There are diminishing returns to experience, but is 5 years of experience really no better than 0 years? If you follow the link, they note that most other studies indicate that "the benefit of that experience appears to plateau after the third to fifth year," which is more plausible than no effect at all.

As for the null result for certification, one plausible explanation is that teachers without certification were hired or kept on because they had otherwise better resumes or job performance than their certified competition. However, we must again ask for evidence.