The responses are predictable online and through social media any time I address teacher quality and policy focusing on teacher evaluation such as my recent commentary on Charleston adopting value-added methods (VAM).
How dare I, some respond, suggest that teacher quality does not matter!
The pattern is exhausting because most responding in indignation first misrepresent what I have claimed and then make the most extreme arguments themselves in order to derail the conversation along their own agenda, usually linked to the charter school movement grounded in teacher bashing and making unobtainable promises.
So let me state here that the central elements of what we know about teacher quality and efforts such as VAM-based teacher evaluation is that teacher quality is not an independent variable (any teacher may be effective for one student and ineffective for another, for example) and, since student high-stakes testing is not designed to measure teacher quality and is more strongly linked to out-of-school factors, VAM is both a horrible technique for identifying teacher quality and, ironically, a guaranteed process for devaluing the importance of teachers.
Teacher quality is unparalleled in importance in terms of student learning, but it is also nearly impossible to measure, quantify—especially through student scores on high-stakes standardized tests.
Teacher quality VAM-pires, then, often have agendas  that are masked by their bluster about teacher quality.
Trying to measure and quantify teacher quality is a mistake; linking any evaluation of teacher quality to student test scores lacks validity and reliability—and VAM discourages teachers from teaching the most challenging populations of students (high-poverty, special needs, English language learners).
Focusing on simplistic and inappropriate measures reduces teacher impact to 10-15% of what high-stakes standardized testing measures; in other words, VAM itself devalues teacher quality.
My informed argument, based on 18 years as a public school classroom teacher and 15 years as a teacher educator and scholar, then, is that we must recognize teacher quality is impacted by teacher preparation, teaching/learning conditions, student characteristics, and dozens of other factors inside and outside of schools—many of which are beyond the control of teachers or students.