Is 'teacher subject knowledge' important?

On Hattie's list, 'Teacher Subject Knowledge' ranks very low - Effect-size 0.09 - almost at the bottom of the list. (See under 'Evidence' tab).

However, this report 'What makes great teaching?' by eminent researchers including Robert Coe and Steve Higgins has been reported as showing that it is top of the list for effectiveness.

Is this contradictory research?  A closer look shows they are talking about different things.  

Hattie is talking about knowledge over and above that needed for the course, for instance, that the teacher has a PhD or industry-experience in the subject.  This has little effect on the quality of learning. (Some experts baffle their students.)

Coe is talking about the material you are teaching.  Here (obviously!) if you don't know the material well, your students' learning suffers.  They use the term 'Content knowledge' (see quote below) which refers not just to the content itself, but also: 

• knowing the subject well enough to explain it in different ways
• being able to answer student's questions
• understanding the difficulties they commonly have in grasping this knowledge 
• etc

So, no conflict - just another example, perhaps, of how the teaching profession does not yet agree on the technical language it uses to describe things!

(Pedagogical) Content knowledge

A number of studies have found a relationship between measures of a teacher’s

knowledge of the content they are teaching and the gains made by their students.

It seems intuitively obvious that ‘Teachers cannot help children learn things they

themselves do not understand’ (Ball, 1991, p5). However, the search for a

relationship between characteristics such as academic qualifications or general

ability and student performance has been rather disappointing: correlations are

typically very small or non-existent (Rockoff et al, 2011).