The academic impact of enrollment in international baccalaureate diploma programs, a case study of chicago public schools

The Academic Impact of Enrollment in International Baccalaureate
Diploma Programs: A Case Study of Chicago Public Schools

Anna Rosefsky Saavedra

Advisor
Meira Levinson

A Thesis Presented to the Faculty
of the Graduate School of Education of Harvard University
in Partial Fulfillment of the Requirements
for the Degree of Doctor of Education

2011


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The Academic Impact of Enrollment in International Baccalaureate
Diploma Programs: A Case Study of Chicago Public Schools

Anna Rosefsky Saavedra

Ad hoc committee
Meira Levinson
Richard Murnane
John Willett

A Thesis Presented to the Faculty
of the Graduate School of Education of Harvard University
in Partial Fulfillment of the Requirements
for the Degree of Doctor of Education

2011


Vita
Anna Rosefsky Saavedra

1994-98

Yale University
New Haven, CT

1998-99

High School Teacher, History
Pinewood School
Los Altos, CA

1999-05

Educational Programs Manager
EF Educational Tours
Boston, MA

2005-06

Graduate School of Education
Harvard University

2006-11

Doctor of Education candidate
Graduate School of Education
Harvard University

2006-09

Teaching Fellow

Graduate School of Education
Harvard University

2009-10

Fulbright Student Scholar
Bogota, Colombia

B.A.
June 1998

Ed.M.
May 2006


© Anna Rosefsky Saavedra
2011
All Rights Reserved


Acknowledgements
Without the support of many people, I would not have been able to begin or finish
this project. I will always be thankful to every one of them for the roles they played in
enabling me to write this dissertation. Even after studying the impact of enrollment in the
International Baccalaureate Diploma Program for past three years, I still thoroughly enjoy
research of this topic and am keen to continue it as part of my future research agenda!
Specifically, I am grateful to Sara Leven from Chicago Public Schools (CPS) for
providing the support necessary to begin this project in the first place and for addressing
my ongoing questions throughout its duration. Thank you as well to Amy Novell for
approving my use of CPS data. Many thanks to Sue Sporte and Vanessa Coca from the

Consortium on Chicago School Research for providing me with the data I use in this
project and for also addressing my ongoing questions.
I am grateful for generous financial support from the Center for the Advancement
and Study of International Education, the Fulbright Foundation, the Harvard Frederick
Sheldon Fund Travel Fellowship and the Harvard Graduate School of Education (HGSE).
Financial resources from these organizations made it possible for me to dedicate a
substantial portion of my time over the past two years to this dissertation.
Several of my doctoral colleagues and friends offered valuable feedback to
previous versions of this dissertation, including Liz Dawes Duraisingh, Rebecca
Holcombe, Qian Guo, Maria Elena Ortega and Justin Reich. I also feel gratitude toward
particularly supportive HGSE professors David Perkins and Eileen McGowan, who, over
my years at HGSE, became friends as well as people for whom I have the utmost respect.
To my dear friend Leila Morsy Eckert, I will forever be thankful the fates threw us


ii
together as statistics partners. I am so fortunate to have gone through HGSE with Leila
and look forward to our annual "road race," no matter where we both happen to be living.
I especially thank my advisors Meira Levinson, Richard Murnane and John
Willett for supporting me through the dissertation, from beginning to end. Individually,
each helped me with specific parts of the process and, in sum, their advice, time and
support were critical to my completion of the doctoral program and this dissertation. I
will miss each of them but will stay in touch and will keep their advice with me wherever
I go. To Meira in particular, I will always be grateful for taking me on as her first
doctoral student and then for guiding me through the doctoral program with enthusiasm
and style. I could not have found a more encouraging, loyal and caring advisor, who will
always by my role model, mentor and friend.
I am also so thankful to my parents and sister. It was only by modeling my
father's work ethic and determination that I was able to successfully finish my
dissertation. For his example and for his support in so many other ways I will always be

thankful. I thank my mother for many, many hours of listening, emphasizing and
understanding. She boosted me up when I was down and her face shone with happiness
and pride when I shared good news. I thank my sister Heather for her unwavering
confidence in my ability to finish my dissertation. She is and has always been there for
encouragement or a celebration. To each one of you, I am thankful for your love and
support, I am so fortunate to have you as my family.
And most of all, I am thankful to my dearly beloved husband, Juan Esteban, who
has played all of the roles I describe above and is my best friend and lifelong partner in
all things. In addition to editing drafts, writing code, and otherwise helping me with the


iii

technical aspects of writing a dissertation, his steadfast love and confidence in me
provided me with the foundation from which I was able to progress through my doctoral
studies and eventually complete my dissertation. He writes at the end of his dissertation
that he is thankful to me for conquering every bit of his dissertation milestone with me. I
could not feel more similarly with regards to my dissertation and all we have
experienced, enjoyed and conquered together.


iv
Table of Contents
Abstract

v

The International Baccalaureate Diploma Program

1

The IB Diploma Program in Chicago Public Schools

4

Research Questions

7

Research Design

8

DATASET
SAMPLE
MEASURES
ANALYTIC METHODS

Results
THE IB ENROLLMENT PROCESS
RQ #1: THE IMPACT OF IB ENROLLMENT ON STUDENTS' ACADEMIC ACHIEVEMENT
RQ #2: THE IMPACT OF IB ENROLLMENT ON STUDENTS' PROBABILITY OF HIGH-SCHOOL
GRADUATION
RQ #3: THE IMPACT OF IB ENROLLMENT ON STUDENTS' PROBABILITY OF COLLEGE
ENROLLMENT
RQ #4: GENDER DIFFERENCES IN THE IMPACT OF IB ENROLLMENT

Threats to Validity

8

9
9
13

17
17
21
22
23
23

24

CHALLENGE #1: SELECTION BIAS
24
CHALLENGE #2: IB ENROLLMENT INCREASES THE PROBABILITY THAT STUDENTS TAKE THE
ACT EXAMINATION
29
CHALLENGE #3: IB ENROLLMENT INCREASES THE PROBABILITY THAT STUDENTS GRADUATE
FROM HIGH SCHOOL
30
CHALLENGE #4: DIFFERENTIAL PROBABILITY OF MISSING SEVENTH-GRADE ITBS SCORES ..31
CHALLENGE #5: EXTERNAL VALIDITY
32

Discussion

32

Cost of the IB Diploma Program Effects

36

Conclusion

38

References

40

Tables and Figures

44

Vita

52


V

Abstract
In this study, I examine whether eleventh-grade students' enrollment in the Chicago
Public Schools (CPS) International Baccalaureate (IB) Diploma Program improves their
academic achievement as measured by their ACT examination scores, probability of
high-school graduation and probability of college enrollment. CPS offers the IB Diploma
Program in thirteen high schools, more than twice any other U.S. school district. Using
data on the IB enrollment status of 20,422 students attending these thirteen high schools
from 2002-2008,1 estimate that IB enrollment increases students' academic achievement

by as much as 0.5 standard deviations and their probability of high-school graduation and
college enrollment by as much as 17 and 22 percentage points respectively. All of my
estimates are highly robust to validity threats posed by self-selection into IB enrollment.
All estimates are greater for boys than for girls. I also calculate that the IB Diploma
Program is a cost-effective way to increase high-school graduation rates.


1
The Academic Impact of Enrollment in International Baccalaureate
Diploma Programs: A Case Study of Chicago Public Schools
Originally founded as a private means for diplomats' children to earn an
internationally recognized high-school diploma, today the International Baccalaureate
(IB) Diploma Program is one of the fastest growing curricular innovations in U.S. publicschool districts. In schools accredited as "IB World Schools" by the International
Baccalaureate Organization (IBO), teachers use IB curriculum and pedagogy to teach a
range of courses that are intended to prepare IB-enrolled students for college. In this
study, I examine whether IB enrollment does in fact prepare students for college.
Specifically, I ask whether enrollment in the IB Diploma Program increases students'
academic achievement as measured by ACT college admissions examination scores,1
probability of high-school graduation and probability of college enrollment and whether
my estimates differ by gender.
The International Baccalaureate Diploma Program
Over the past fifteen years, the number of U.S. schools that implement IB
programs has increased nearly tenfold, from 133 in 1994 to 1,218 in 2011. By 2009,
public schools offered over 90% of U.S. high-school level IB programs (IBO, 2010).
What began as a program that a few private schools offered to wealthy children, today
reaches a much broader student audience.
This growth in IB implementation is based predominantly on the program's
perceived academic rigor and success as a college-preparatory intervention (Byrd,
Ellington, Gross, Jago, & Stern, 2007; Cech, 2008; Mathews & Hill, 2005). This
1

The ACT organization administers the ACT college admissions examination, which
includes English, mathematics, reading and science sections (ACT, 2010).


2
reputation, augmented through recent governmental support, suggests that the growth
trend will continue. For example, through the reauthorization of the Elementary and
Secondary Schools Act, the U.S. Department of Education has proposed to fund IB
programs as part of its objective to "increase access to accelerated learning opportunities"
(U.S. Department of Education, 2011a, 29). State-level education departments are also
promoting IB implementation as part of their "Race to the Top" strategies (e.g. FL, MA,
ME, NH; U.S. Department of Education, 201 lb).
The IB school-level accreditation process and subsequent requirements for
students and teachers are rigorous (Byrd et al., 2007; Mathews & Hill, 2005; U.S.
Department of Education, 2010). To earn accreditation from the IBO, schools must
demonstrate adherence to IBO's curricular, pedagogical, mission-based and ongoing
professional-development requirements as stated in the most current version of the
"Handbook of procedures for the Diploma Program" (IBO, 2009). Typically, the IB
accreditation process requires extensive faculty and staff participation, takes several years
to complete and, if successful, results in authorized "IB World School" status. IB WorldSchool teachers must use IB curricular and pedagogical materials to teach IB courses, and
participate in ongoing professional development and self-review processes.
From a financial perspective, as of 2010, IB World-School accreditation costs
schools approximately $9,500 per year during the two years of candidacy. Thereafter,
accredited schools pay annual fees—of $9,6000 USD in 2010—to access IB curriculum
and support. These fees do not differ according to the number of students estimated to
enroll in IB courses, once the school is accredited. IB World Schools offering the
Diploma Program must also provide a way for students to take the IB Diploma

3
examinations at the end of their twelfth grade. Per-student, as of 2010, registration to take
IB examinations costs $135 and the examinations themselves cost $92 per subject (IBO,
2010).2 In some schools, the students pay for the examination fees, while in others, the
schools themselves pay the fees.
Students enroll in the official IB Diploma Program in eleventh grade. Over the
course of their final two years of high school, to be eligible to earn the IB Diploma,
students must: enroll in six core IB courses and the IB Theory of Knowledge (TOK)
epistemology course; participate in the weekly Creativity, Action and Service (CAS)
requirement; and write a 4,000-word "extended essay." They must also score above a
defined threshold on IBO-created and administered examinations in six subjects
including languages, social studies, experimental sciences and mathematics. 9,000 IBcertified examiners, in 121 countries worldwide, assess the examinations (IBO, 2010).
Despite the IB program's rapid expansion, little is known about whether
enrollment in the IB Diploma Program improves students' academic outcomes, including
their high-school academic achievement, probability of high-school graduation and/or
subsequent probability of college enrollment (i.e.: Foust, Hertberg-Davis & Callahan,
2009; Jackson, 2010; Kyberg, Davis & Callahan, 2007; Roderick, Nagaoka, Coca &
Moeller, 2009). To date, only one study has examined the relationship between IB
Diploma Program enrollment and students' high-school academic achievement and
subsequent college entry (Roderick et al, 2009). The purpose of that study, however, was
to examine whether Chicago Public Schools' (CPS) students who enrolled in the IB
Diploma Program, Advanced Placement or other honors-level courses, then enrolled in

2

The IBO tends to increase its fees on an annual basis.


4
colleges that were appropriately competitive. In contrast to the present study, according

to Roderick et al, "[their] report is not intended to be a rigorous evaluation of selective
enrollment schools, IB programs or AP initiatives (2009, 2)."
The IB Diploma Program in Chicago Public Schools
In this study, I examine whether eleventh-grade CPS students' enrollment in the
IB program improves their academic achievement, probability of high-school graduation
and probability of college enrollment. CPS is an ideal setting in which to address this
issue for three reasons. First, CPS, at the forefront of IB implementation nationwide,
offers the IB Diploma Program in thirteen high schools, more than twice any other U.S.
school district.3 Next, the system retains complete historical records of its students' IB
enrollment status, achievement test scores, high-school graduation status and collegeenrollment information. Finally, since the average CPS student is low income and
minority, evidence documenting the IB enrollment impact in this setting would enhance
our knowledge of ways to improve secondary education for disadvantaged urban youth.
In the early 1990s, levels of academic achievement in most CPS high schools
were dismal. Ninth- and eleventh-grade students' achievement scores revealed academic
performance that was, on average, more than a year behind grade level when compared to
national averages (Chicago School Reform Board of Trustees, 1999). The district's highschool graduation rate in 1995 was less than 50% of an entering cohort, relative to the
national average of 76% (Heckman & LaFontaine, 2010).4 Among Illinois students who

3

www.ibo.org. retrieved 5-30-09. Indianapolis currently has six high school level IB
programs, followed by five in Philadelphia and four in St. Paul, Minnesota.
See Heckman & LaFontaine (2010) for a discussion of U.S. high school graduation
rates.


5
took the 1994-1995 ACT college-entrance examination—50% of CPS seniors and 57%
statewide—CPS students lagged behind their peers statewide by nearly five points out of
a maximum score of thirty-six (Rice, 1995).

Boys who attended CPS neighborhood schools fared particularly poorly. For
example, in the population of CPS students who were thirteen years old in 1998, only
39%o of African-American, 51% of Latino and 55% of White boys had graduated from
high school by 2004, when this cohort was nineteen years old. In comparison, 57% of
African-American girls, 65% of Latino girls and 71% of White girls in the 1998 cohort
graduated by 2004 (Allensworth, 2005). Arguably, the need for school reform in CPS
was even greater for boys than for girls.
From 1981 through 1997, Lincoln Park was the only public, non-magnet high
school, out of more than 90 in CPS, to offer the IB Diploma Program. From its inception,
Lincoln Park IB students' academic achievement far surpassed that of students from other
neighborhood high schools and even from most of the CPS selective-enrollment high
schools (Roderick et al, 2009).
Driven by Lincoln Park's IB success and support from the Chicago mayor and
business community, beginning in 1997, CPS increased rapidly the number of IBaccredited programs offered in Chicago neighborhood high schools. CPS's primary goal
for widely implementing the IB Diploma Program was to increase predominantly lowincome and minority students' access to academically rigorous curricula in neighborhood
high schools and, thereby, to increase their academic achievement and college
preparedness (Spittle, Leven, & Roderick, 2008).


6
The thirteen CPS high-school level IB programs serve primarily low-income,
minority students: CPS administrative data show that 65% of CPS students enrolled in IB
are eligible to receive free or reduced price lunch and 74% are racial minorities. Among
non-IB (eleventh grade) students in the district, in comparison, 78% are eligible to
receive free- or reduced-price lunch and 85% are racial minorities. Therefore, both
groups are majority-minority and low-income, but compared to non-IB CPS students, the
probability is greater that CPS IB students will be White and higher income.
The IB Organization (IBO) does not dictate how schools determine student
eligibility for enrollment in IB programs. The CPS policy is to require all students who
think they would like to enroll in the official eleventh- through twelfth-grade IB Diploma

Program in their geographically determined neighborhood high school to apply in their
spring of eighth grade so that they can participate in an IB-preparation track during ninth
and tenth grade. The nature of the preparation track differs by CPS high school. Several
offer accredited IB-preparation programs (the IB Middle Years Program), while others
offer AP courses or other honors classes.
CPS requires that students submit their seventh-grade Illinois Test of Basic Skills
(ITBS) mathematics and reading scores as part of the IB application requirement. For the
first few years of IB Diploma Program implementation, the stated CPS policy was to
admit only students scoring above the sixtieth percentile on both tests. This policy has
evolved such that today the "unofficial" school-level policy is to admit only students
scoring above the fiftieth percentiles on both tests.


Research Questions
Currently, policy makers, administrators, teachers, parents and students lack
credible evidence on whether enrollment in the IB Diploma Program increases students'
academic achievement, probability of high-school graduation and probability of college
enrollment and whether effects might differ by gender. Such evidence could inform
decisions about future investments in the IB Diploma Program throughout Chicago, and
more broadly, in U.S. public schools. This deficiency motivates my research questions:
•

RQ#1:
A) Does CPS students' enrollment in the IB Diploma Program in eleventh grade
increase the probability that they will take the ACT college entrance examination?
B) Does CPS students' enrollment in the IB Diploma Program in eleventh grade
improve their high-school achievement, as measured by their ACT scores?

•

RQ #2: Does CPS students' enrollment in the IB Diploma Program in eleventh
grade increase the probability that they will graduate from high school?

•

RQ #3: Does CPS students' enrollment in the IB program in eleventh grade
increase the probability that they will enroll in a two- or four-year college at some
point during the first two years after their graduation from high school?

•

RQ #4: Do potential effects of IB enrollment on students' probability of taking the
ACT examination, academic achievement as measured by ACT scores,
probability of graduation from high school and probability of enrollment in
college, respectively, differ by gender?


8
Research Design
Dataset
The data that I analyze here contain information merged from two sources, from
the Chicago Public Schools (CPS) themselves and from the National Student
Clearinghouse (NSC), a non-profit organization that verifies students' enrollment in, and
graduation from, 92 percent of U.S. colleges (National Student Clearinghouse, 2011).5
The CPS dataset contains records for the 20,422 students who attended one of the thirteen
CPS high schools that offered the IB Diploma Program to the 2002-2003, 2003-2004,
2004-2005 and 2006-2007 eleventh-grade cohorts.6 It contains measures of students' IB
enrollment status, previous academic performance, gender, race/ethnicity and family
income, as well as students' mathematics, English, reading and science scores on the
ACT test, and indicators of whether they graduated from high school.

Data on students are included in the NSC dataset if they enrolled in a college that
is included among the ninety-three percent of colleges whose admissions data is collated

5

The majority of CPS students who enroll in college attend postsecondary institutions
located in Illinois (Roderick, 2006). According to the NSC's Assistant Director of
Research Services, "the NSC post-secondary enrollment data coverage for students in
Illinois is higher than the national average of 92%" (personal correspondence, February
23, 2011). By cross-referencing NSC data with CPS exit survey data, Roderick, Nagaoka,
Allensworth, Stoker, Correa, & Coca (2006) estimate that the NSC data "misses
approximately 5 percent of CPS graduates who may be enrolled in college" (cited in
Roderick, 2006, footnote 56). Roderick et al also characterize the Illinois colleges for
which the NSC does not verify enrollment as "primarily local proprietary and technical
institutions" (2006). Contingent upon the assumption that compared to IB students, the
probability is greater that non-IB students will attend local proprietary and technical
institutions, missing NSC enrollment data could contribute to a slight understatement of
the percentage of non-IB students who enroll in college.
I do not have access to records for the 2005-2006 eleventh grade cohort. I have no
reason to believe that the results of my analysis would differ if I could include those
records in my full sample.


9
in the National Student Clearinghouse at some point during the first four semesters postgraduation from high school. I located fifty percent of the 20,422 students in the CPS
sample in the NSC dataset, indicating that at least half of CPS students in my full sample
enrolled in two- or four-year college for at least one semester during the first four
semesters after their graduation from high school.

Sample

My full sample of CPS students (n=20,422) contains every eleventh-grade student
in each of the thirteen CPS high schools that offered the IB program to the eleventh-grade
cohort in academic years 2002-2003 (n=5042), 2003-2004 (n=5,097), 2004-2005
(n=5,181) and 2006-2007 (n=5,102). I use data from these years because college
enrollment data is available on each of these cohorts. I combine four annual cohorts of
students to improve the statistical power of my analyses and enhance the external validity
of my findings.

Measures
Outcomes
In Table 1, I present descriptive statistics on my outcomes, question predictors
and covariates for the full sample of students included in my CPS dataset (n=20,422).
The first of my outcome variables is a measure of student achievement, which I label
ACT, and which I formed by conducting a principal-components analysis (PCA) of
eleventh-grade students' mathematics, English, reading and science scores on the ACT
college entrance examination. The resulting continuous variable is a symmetrically
distributed, weighted linear composite with an estimated Cronbach's alpha internal-


10
consistency reliability coefficient of 0.89. In total, 2,667 students—1 percent of IB
students and 14 percent of non-IB students—are missing on at least one of the four
constituent achievement tests and thus are also missing on the academic achievement
outcome itself. After compositing, for ease of interpretation, I standardized the obtained
first principal component score to a mean of zero and a standard deviation (S.D.) of unity
in my analytic sample of students (n=14,368) who were not missing seventh-grade
mathematics or reading scores nor one of the ACT subject-level tests. Students take the
constituent achievement tests during the late spring of their junior year, and so my
estimated "treatment effects" represent the impact of only one year of official IB
enrollment on the academic-achievement outcome.

«Insert Table 1 h e r e »
My second outcome, HSGRAD, is a dichotomous indicator that I coded 1 if
students graduated on time from high school with a regular CPS degree or an alternate
Illinois degree (0 otherwise).7 I possess this information for every student in the full
sample, of which 99 percent of IB and 78 percent of non-IB students graduated from high
school.8
My third outcome, COLLENROLL, is a dichotomous indicator that I coded 1 for
students who enrolled in either two- or four-year college for at least one semester during
one of the first four semesters post-graduation from high school (0 otherwise). The NSC
only matches CPS high-school graduates to its college enrollment database. Therefore,
7

The requirements for the CPS high school degree are more demanding than the
requirements for an Illinois degree. My data does not indicate whether students later
earned the General Education Development equivalency degree (GED).
8
Though these proportions seem high given the disadvantaged context, these students
persisted beyond ninth grade, the year in which CPS students are at the highest risk of
dropout (Allensworth, 2005).


11
students' COLLENROLL status is conditional on whether they graduated from high
school, such that the COLLENROLL=0 for students for whom HSGRAD=0. 83% of IB
students and 48% of non-IB students enrolled in college.
If IB enrollment affects the probability that students will graduate from high
school, then relative to non-IB students, a greater proportion of IB students will be
included in the sub-sample of students that I use to estimate the impact of IB enrollment
on students' probability of college enrollment. The overrepresentation of IB students in
this sub-sample would then contribute to biasing my estimation of the impact of IB

enrollment on students' probability of college enrollment. I discuss this potential threat to
a causal interpretation of my estimates of the impact of IB enrollment on students'
probability of college enrollment in the "Threats to Validity" section of this paper.
Question predictor
I define my question predictor, IBTREAT, as whether students enrolled in the core
IB Theory of Knowledge course for at least a semester (l=yes, 0=no). According to the
CPS IB Diploma Program Administrator (personal correspondence, July 6, 2010),
students typically only enroll in TOK if they are attempting to fulfill the IB program
requirements and only rarely as a way to take honors courses in a piecemeal fashion. The
students that enroll in TOK, on average, enroll in eleven IB courses (SD=4), confirming
this characterization of TOK course enrollment. According to my definition, seven
percent (1,403 students) of the full sample of 20,422 students was enrolled in the IB
treatment.
Covariates
I include in my analysis students' seventh-grade mathematics (MATH) and


12
reading (READ) test percentiles, which are based on their national percentile rank on the
ITBS test, as measures of their previous academic achievement. CPS administrators use
these test percentiles to determine IB eligibility. 26% (5,261) of students in my full
sample and 20% (279) of IB students are missing these scores. I eliminate students
missing on MATH or READ, leaving an analytic subsample of 15,148 students. As I show
in Table 1, among IB students, the mean ITBS mathematics-achievement percentile is
79.47 (SD=16.18) and the mean reading-achievement percentile is 73.86 (SD=17.71). In
comparison, the non-IB mean mathematics percentile of 50.27 (SD=24.95) and mean
reading percentile of 47.13 (SD=22.97) are each almost thirty percentiles lower.
I also include as covariates in my analyses measures of students' demographic
characteristics, including their gender, family income and race. I coded MALE as 1 for
the boys, who make up 48% of non-IB students and 35% of IB students. I represent

family income using LUNCH, a dichotomous indicator that I coded 1 for students who
were eligible to receive free or reduced-price lunch. Over all, 65% of IB students were
eligible, compared to 78% of non-IB students, indicating that on average, both non-IB
and IB students are from low-income families, though the probability that non-IB
students are from poorer families is greater. I represent student race using a vector of
dichotomous indicators that describe whether a student was Asian-American (ASIAN),
African-American (AFAM), Latino (LATINO), Native American or Alaskan American
(NATIVEAM), or White (WHITE), dropping the last to create a reference category.
Among all IB students, 30 percent are Latino, 28 percent are African-American, 26
percent are White, 17 percent are Asian-American and less than 1 percent are Native


13
American or Alaskan, while the corresponding proportions are 42%, 37%, 15%, 6% and
under 1%, respectively, among non-IB students.
In following section, I use the generic variable name STUDENTDEM to represent
the vector of covariates that record students' ITBS test percentiles, gender, family income
and race.
Analytic Methods
My objective is to estimate the causal impact of enrollment in the IB Diploma
Program on CPS eleventh-grade students' academic achievement, probability of highschool graduation and probability of college enrollment. I use the same analytic strategy
to address each of my research questions, which I describe for outcome, Y, for student /' in
high school j in cohort c. I first assume selection into the IB Diploma Program based on
students' observed characteristics by using a. propensity score approach to estimate the
impact of IB enrollment on my three measures of students' academic success. Then, as I
discuss in the "Threats to Validity" section of this paper, following Rosenbaum (2002), I
test the sensitivity of my estimates to different levels of selection bias.
In fitting my propensity-score selection model, I use logistic regression analysis to
model students' probability of enrolling in the IB program as a function of a set of
measured characteristics that I have determined theoretically should predict enrollment

(Murnane & Willett, 2011 311). I include students' seventh-grade mathematics and
reading test percentiles as the principal predictors in my propensity score selection model
because I anticipate that these test results will be strong predictors of whether students


14
enroll in the IB Diploma Program, given the CPS IB-admission policy. 1 also include an
9

indicator of gender because CPS boys' tendency to fail classes, earn lower grade-point
averages and drop out of school at higher rates than girls (Allensworth, 2007) suggests
that they have a lower theoretical probability of CPS IB enrollment eligibility than girls. I
include measures of race and family income as predictors because historically IB has
attracted White, Asian-American and economically advantaged students, while AfricanAmerican, Latino and economically disadvantaged students have not had the academic
preparation necessary to enter the rigorous Diploma Program (Burris, Welner, Wiley, &
Murphy, 2007).
I therefore fit the following logistic regression model (1) to represent the
probability that student / in schooly in cohort c chooses to enroll in the IB program:
¥r(IBTREATlJC = 1) =
(I)

-P0+P,STUDENTDEMIJC+SJ+\

'

e

I include school (S,) and cohort (Xc) fixed effects to allow for the probability that student i
will enroll in IB to differ by school and cohort. The fixed effects address the concern that
schools' and cohorts' unmeasured characteristics might contribute to determining

whether students will enroll in IB.
Based on the fitted propensity score selection model, the propensity scores
(PHAT) are the predicted probabilities that students will enroll in the IB program. Once I
have obtained PHAT for each of the students in my analytic sample, I follow Imbens and
Wooldridge (2009) in eliminating from my analytic sample all non-IB students with
9

I use quadratic functional forms of the mathematics and reading percentiles because this
more flexible form ought to be a better fit for IB students' ITBS scores. Estimates of the
IB enrollment impact are statistically indistinguishable in models in which I include
respective nested linear, quadratic, cubic and quartic functional forms of the ITBS
percentiles.


15
estimated propensity scores that fall outside of the range of estimated propensity scores
for the IB students. This process aligns the IB and non-IB groups explicitly on the
selection model covariates, ensuring that the non-IB students in my comparison sample
are as similar as possible in terms of measured characteristics to IB students. The
resulting comparison sample includes 11,592 student records.10
Then, following Hirano, Imbens and Ridder (2003), in my subsequent analysis of
program impact, I use PHAT io provide an inverse-probability weighting, IPW, for each
student in my comparison sample. I create the weights as follows: For IB students:
IPW=\IPHAT; for non-IB students: IPW=\l(l-PHAT). The purpose of this weighting
strategy is to allocate the least weight to the IB students with the highest propensity for
selection into IB and the greatest weight to the IB students with the lowest propensity for
selection into IB (and vice versa for non-IB students) in order to impose identical
(observed) covariate distributions for students who enrolled versus did not enroll in the
IB program.
The final step is to fit my weighted least squares (WLS) estimation model (2),

which represents the relationship between my academic outcomes and membership in the
IB treatment, weighting each student i in school j in cohort c by his or her IPW as
follows11:

10

1 define the 3,469 students who were missing on an IB treatment status as being non-IB
rather than dropping them. My rationale is that according to model (1), the distribution of
propensity scores for students missing on IB status (mean=0.053, SD=0.113) is to the left
of the distribution of propensity scores for students who explicitly did not choose the IB
option (mean=0.065, SD=0.113). Therefore, students missing on IB status, if anything,
look more like non-IB students than IB students, for whom the mean propensity score is
0.294 (SD=0.210).
11
In practice, I weight each observation by the square of the weights (Imbens &
Wooldridge, 2009).