States Tackle Education's Holy Grail:
Measuring teacher
effectiveness has long been a complex—and often divisive—issue in the education
policy world. Nonetheless, valid measures of effectiveness remain necessary to
help policymakers make informed decisions about compensation, tenure,
retention, and professional development for educators.
Over the last few years,
more than 30 states have dramatically reformed how they evaluate teachers and
principals. Most of these new evaluation systems combine data on objective
evidence of student learning (such as value-added modeling) with other
measures, such as teacher observation ratings and student satisfaction surveys.
However, what remains
uncertain is how successfully school districts can use these data to implement
a single, combined measure of effective teaching and whether they can
distinguish among teachers and principals of varying quality.
The appeal of using a
combined measure is that these would be easier to interpret than multiple
indicators and could facilitate communication to the public. Combined measures
have been used to promote accountability and evaluate performance in numerous
sectors, including health care, universities, and local and national
governments.
That said, using a
combined measure could invite simplistic or misleading policy conclusions if
the measure is misinterpreted or poorly constructed. Similarly, a combined
measure could be misused if the process of constructing it is not transparent
or not based on sound principles.
There is limited
empirical research to guide policymakers on how best to develop combined
measures of teacher effectiveness, but a new RAND report sheds some light on
this challenge.
Researchers examined data
from the Measures of Effective Teaching Project to estimate how much unique
information resides in each of the indicators of effective teaching and to
understand the trade-offs of different ways of combining indicators. They found
that while there are some common aspects to effective teaching, each of the
indicators contains a fairly large amount of unique information. This has
important policy implications: the more unique information, the more it matters
how the indicators are combined into a single measure.
Weighting schemes, which
determine the extent to which each indicator is factored into the combined
measure, play an important role. Depending on what the goals are for the
combined measure, some weighting schemes may be more or less effective at
predicting teachers that achieve the goal. If the goal of the combined measure
is to only predict a teacher's student achievement gains, then the student
achievement gain measure should receive over 80% of the weight. However, a
singular focus on student achievement gains could lead teachers to narrowly focus
on this aspect of teaching and ignore other valued outcomes. If the goal is for
students and teachers to meet a broader set of objectives, then scores on each
indicator should be combined with a relatively equal weight. The research also
found that an equally weighted composite is more stable year to year.
Creating a combined
measure of teaching effectiveness has obvious appeal, since no single method
provides a complete picture of a teacher's effectiveness. While clearly there
are significant challenges and trade-offs associated with this undertaking,
research can help guide the way.
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