Everyday Americans and politicians alike think class-size reduction is a key to any plan to improve education in America. I used to agree. Then I ran the numbers. I've since come to the conclusion that class-size reduction is a $40 billion mistake. Allow me to explain.
Though much research has shown (and
common sense confirms) that teacher quality is the key variable when it comes
to student improvement, teacher quality is hard to measure. And without
accurate measurement, it is impossible to compare the impact of the various
known components of teacher quality to the impact of other seemingly helpful
interventions such as reducing class-size, instituting after-school programs,
hiring additional school counselors etc. Lacking a scale for comparison we
can't evaluate financial trade-offs and politicians are likely to go for
popular, feel good programs of which class-size reduction is the American
favorite. But new research has given us exactly the tools we need to make
precise comparisons and the financial cost-benefit analysis this research makes
possible is simply damning for class-size reduction.
The best source of this information
is a new study called "How
and Why Do Teacher Credentials Matter for Achievement" (Clotfelter,
Ladd, and Vigdor 2007). Using value-added methodology*, this study took 10
years of data for every student and teacher in the state of North Carolina and
used it to analyze the impact of various factors -- including teacher
experience and credentials, student socio-economic background, and class-size
-- on student achievement. Though the study was not new in conception, its
massive data set suggests that its results may be the most reliable to date.
Of particular interest are the
following results (for math achievement**):
Input
|
Increase in student achievement
|
The first 3 to 5 years
of teacher experience:
|
7.2% - 9.1% of a
standard deviation
|
(vs. a brand new teacher)
|
|
Teacher having a
regular teaching license:
|
3.3% - 5.9% SD
|
(vs. having an emergency license)
|
|
Teacher same race as
student:
|
2.0% - 2.9% SD
|
-All additional years
of teacher experience:
|
2.0% - 2.8% SD
|
(beyond 5 up to 27)
|
|
Teacher is National
Board certified:
|
2.0% - 2.8% SD
|
Teacher scored 1 SD
above average on
|
1.1% - 1.5% SD
|
a teaching licensure exam:***
|
|
Reducing class-size by
5 students per teacher:
|
1.0% - 2.5% SD
|
Teacher attended a
competitive college:
|
0.7% - 1.0% SD
|
(vs. attending an uncompetitive college)
|
|
Reducing class-size by
1 student per teacher:
|
0.2% - 0.5% SD
|
Teacher has an
advanced degree:
|
–0.3% - 0.2% SD
|
(usually MA)
|
The numbers above represent the
amount of improvement that students experienced when given the input listed.
It's very important to note that the results above are measured in percent of a
standard deviation, not in percent of a test score. If you aren't statistically
inclined enough to interpret what standard deviations mean, don't worry; the
percentages above still tell us the relative impact of these factors, which is
the crucial factor when comparing trade-offs.
Comparing those relative factors
you'll notice several things. First of all, teacher experience and licensure
rank near the top of the list while class size reduction and teachers having
masters’ degrees rank at the bottom. But more important than the ranking is how much smaller of an impact class-size
reduction has than some of these other factors. For example, having a teacher
who is not a novice (7.2% - 9.1% SD) exerts an influence 3½ to 7 times greater
than the impact of reducing class size by 5 students (1.0% - 2.5% SD).
But the really mind-blowing results
come when you start comparing class-size reduction to giving students a teacher
with a reasonably good (though not unlikely) combination of teacher credentials
(estimated by adding the relevant values in the table above). Clotfelter and
Ladd do their own estimates of this sort and come up with a combined effect
size of 15% - 20% SD for math (and 8% -12% SD for reading) of a well-credentialed
teacher.
The first time I read this portion
of the study I said to myself, "Yes, class size is less important,” but
that finding is not particularly novel to anyone who follows this sort of
research. However when I decided to actually compare how much less important class size is my jaw dropped. The effect
size of teacher credentials is 8 to 10 times that of a major class size
reduction in math and 6 to 8 times as big in reading.
To really understand the importance
of these differences in scale you need to look at a few statistics in order to
put a dollar amount next to a policy choice. America currently has a nationwide
student to teacher ratio of about 16 to 1 (roughly 50 million public school
students divided by roughly 3.1 million teachers). The average teacher salary
in America currently floats around $48,000, which means we are spending roughly
$150 billion dollars a year on teacher salaries. Cutting class size in half
would require doubling the number of teachers and therefore doubling the amount
we spend on teacher salaries, putting the figure at around $300 billion. (This does
not even account for the increased costs of benefits, additional classrooms, and
other factors needed to enact such a decrease in class size.)
The next important question is,
"What would we get for such a monumental expenditure?" Based on the
results in North Carolina we can estimate. If class size reduction has a linear
effect-size (granted an assumption worth exploring) cutting it in half (i.e.
reducing it by 8 students per teacher nationwide) would achieve an effect size
in math eight times that of reducing it by one student per teacher, in other
words 1.6% to 4% SD. To give you a sense of the meaning of that effect size, it
is in the same ballpark as giving students a teacher who is of the same race
(about 2% - 3% SD). The former intervention would cost nationwide about $150
billion dollars (or better than half of the $250 billion that all the states
combined spend on education) and the other would cost roughly nothing.
The analysis so far has not compared
the impact of class-size reduction to the impact of teacher effectiveness
differences that have nothing to do with credentials. Put simply, quite a lot
of research has shown that there is a huge difference in the performance of
teachers that is not attributable to differences in credentials. (See for
example the seminal "Teacher Effects on Longitudinal Student
Achievement" Jordan, Mendro, and Weerasinghe 1997 or more recently Kane,
Rockoff and Staiger's work published in simplified version at "Photo
Finish" in Education Next and available at
http://www.hoover.org/publications/ednext/4612527.html).
It's not possible to directly
compare the effect sizes from the North Carolina data set to those in these
other studies, but what these other studies have shown is that in much the same
way that teacher credentials are an order of magnitude more important than
class size, teacher talent is an order of magnitude more important than teacher
credentials. This subject is actually worthy of another post entirely. The
simple upshot is that if a direct comparison shows that the cost-benefit ratios
of reducing class size and improving teacher credentials are in different
categories, the cost-benefit ratios of reducing class size compared to increasing
the concentration of teaching talent are on different planets.
So how about instead of setting a
goal of halving class size, requiring us to spend $150 billion annually, we
instead set a goal of doubling teacher pay, so that average is around $100 K
and simultaneously impose a ruthlessly competitive tenure process where districts
only offer tenure to teachers with dramatically
high value-added?
For those who think that all this
discussion of halving class size (i.e. cutting it by 8 students per teacher) is
a crazy hypothetical, it's worth looking at the trend over the past 30 to 40
years. Since 1970 class size has gone down in America by about 6 pupils per
student, a roughly 27% decrease. There is no indication that teacher quality
has gone up by 27% or even 2.7% in that same time period.
Judging by the education news media,
I must be the only person running these numbers. Every other news article,
position statement by a politician, teacher blog, or opinion on the street,
assumes that reducing class size is a high priority and should be done whenever
possible. But it is quite simply the most misplaced priority out there and no
one seems to be drawing any attention to this fact. The mild experiments in
different versions of performance pay, about which there have been such intense
fights, represent just peanuts compared to the money that has been sunk, and
that prevailing sentiment proposes to keep sinking, into class-size reduction.
Of course increasing the teacher
talent pool would require recruiting a large number of new teachers. So here's
a proposal of what we could do with the money we would have saved from not
decreasing class-size. How about completely free college for anyone who meets a
high academic threshold (3.5 GPA, 700 or higher Math SAT) and commits to
majoring in math or science and then teaching math or science in a low-income
school for 4 years after college? (I mention these hypothetical credentials
based on the 7% of a SD increase you get from having a teacher with licensure
test scores not 1, but 2 SD’s above the mean). This approach to teacher
recruitment would also help 100,000 students a year with college access and in
particular could offer high-achieving low-income students a powerful
opportunity to attend college while serving the communities from which they
come. (I imagine this might have the side effect of improving the
student-teacher racial matching mentioned above, which was measured as having
an effect size similar to that of halving class size.)
If we assume a good state college
costs $20,000 a year (tuition, room, board, books, everything) for four years
and say we had 100,000 takers for our national program each year, it would only
cost us $8 billion for each cohort (or roughly half of current Title I costs).
If on the other hand we spent the same $8 billion reducing class size we could
hire about 160,000 new teachers and achieve a nationwide decrease in class size
of about 0.8 pupils per teacher.**** Based on Clotfelter, Ladd, and Vigdor’s
numbers the latter intervention would likely produce a nationwide effect size
of less than 0.2% to 0.5% of a SD;
(0.2% – 0.5% of SD would occur if we achieved a full 1 pupil reduction in class
size). Not 2% - 5% SD, but 0.2% - 0.5% SD. For 8 billion dollars.
As I mentioned above, states and school
districts have actually enacted this horrendously inefficient second
intervention. If we simply undid the 6 pupil per teacher, nationwide class-size
decrease of the past thirty years, which would require laying off roughly
830,000 teachers nationwide (about 27%), and put us back to our previous
roughly 22 to 1 ratio (not terrible), we would have an additional $40 billion
or so a year to spend (830,000 teachers times $48,000 per year in salaries). If
instead of teacher recruitment through financial aid we wanted to use this
money to increase teacher salary, it would allow us to raise salaries for the
remaining two and a quarter million teachers by almost $18,000 a year across
the board. That's a raise that people considering teaching would notice.
If districts wanted to be more
strategic than proposed above, we could raise salaries for 75% of those
teachers (about 1.7 million of them) by $10,000 across the board ($17 billion
total) and raise salaries for the top quartile (roughly 560,000 teachers) by approximately
$40,000 per teacher ($23 billion total). Frankly, I would be for this even if
top quartile was determined not by value-added, but by peer-review and
principal evaluations since I believe we'd still be getting enough overlap between
actually good teachers and people being highly compensated. We could then have
those $100K a year teachers that so many of us dream of (or dream of being) and
not just a few of them.
Maybe we could do all this without
laying off any teachers if we just don’t replace the almost 1 million who are
nearing retirement. But to do that we’d have to convince all of America to give
up our most beloved, and misguided, policy intervention. We'd have to convince Americans
that the bottom line in improving student achievement is teacher quality not teacher quantity. We'd have to convince them that there is a real trade-off
being made and that every time you spend money reducing class-size you are not spending it producing, recruiting
and retaining effective teachers. We’d have to convince them that bigger
classes are actually better for education.
--Dewey
*Value-added methodology is a way
of evaluating student-progress as opposed to just absolute test scores. To radically
oversimplify, if a teacher takes a student who is scoring about a 60 on an exam
at the beginning of a year and teaches that student to the level where she
scores an 80 on the same (or a very similar) exam by the end of the year that
teacher has added value of 20 points. If another teacher took a student who was
scoring 85 and took her to scoring 90 that teacher added 5 points. Even though
the second teacher's student scored significantly higher, we would say the
first teacher did a better job helping her student grow, in fact a dramatically
better job. This is a very gross oversimplification
of value-added, but it conveys the basic idea. Most value-added models take
into account various other factors besides just the student's initial test score,
in order to not hold teachers accountable for factors outside of their control
that differentiate their students from the students of other teachers.
**The fact that these scores are
for math, not reading is actually quite important. In general the impact that
school-based factors have on reading tests scores is significantly lower than
the impact that school-based factors have on math scores and conversely the
impact of non-school factors such as parental education levels is much bigger
for reading than for math. Given that parents typically interact with their
children more through language than through math, it makes intuitive sense that
parental (and peer or community) impact on language skills would be larger than
parental impact on math skills. The relevant effect sizes of class-size
reduction on reading achievement are 1.0% - 2.0% SD for a reduction of 5
students per teacher.
***Scoring one standard deviation
higher than average would mean you scored in roughly the top 16% of test
takers. Scoring two standard deviations higher than average would mean you
scored in the top 2% of test takers.
****Figures come from the following
rough calculation: Current ratio: 50 million current public school students
divided by 3.1 million current public school teachers = 16.1 students per
teacher. Ratio after spending $8 billion on a class-size reduction initiative: 50
million current students divided by 3.26 million teachers = 15.3 students per
teacher. Difference: 16.1 – 15.3 = 0.8 student per teacher reduction. This
assumes that the new Math and Science corps replaces retiring teachers as
opposed to being added to the current number of teachers.
Post-script:
The one thing that this analysis
doesn't address is the potential of non-linear effect sizes for class-size
reduction. Clotfelter et al. actually found striking non-linearity in the
effect size for increased teacher test scores with a teacher scoring 1 SD
better than average producing barely 1% SD increase in student achievement, but
a teacher scoring 2 SD better than average on the licensure test producing a
whopping 7% SD increases. Frankly, non-linear effect sizes for class-size seem
more likely than not. At the extremes it is obvious that teaching 40 elementary
age children with even mild discipline issues starts to become ridiculous, and
teaching a class of 4 children is essentially a form of tutoring. At some point
in the future I'm going to come back to this issue. I'll just say for the
moment that the kind of changes in class-size discussed in this paper i.e.
changes of 6 pupils per teacher, are not creating these extreme cases in which
we would predict severe non-linearity. More on this in time to come.
