Answer: We do not know, but the factors school districts use to hire teachers, notably credentials and certifications, do little to indicate teacher quality. I am sure you knew that already.

But in case you forgot, Economist Andrew Leigh of the Australian National University recently published a bonzer study of teacher quality and credentials.

Interesting findings include:

1. A 90th percentile teacher can achieve in half a year what a 10th percentile teacher can achieve in a full year. Thus, raising teacher effectiveness may be at least as cost-effective policy reform as reducing class sizes.

2. Teachers with a masters degree or some other further qualification obtain lower test score gains than teachers without these additional qualifications.

3. Female teachers have larger test score gains in literacy than male teachers.

4. Age and experience are positively related to student test score gains.

5. Yet while there are some systematic patterns, 99 percent of the variation in teacher performance remains unexplained by differences in teacher demographics.

Bottom line: As my friends at the NC Public School Forum (H/T) point out, “The best teachers appear to be good at their jobs because of innate factors like personal drive, curiosity and ability to relate to students.”