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DOI: 10.1126/science.1165919
, 122 (2009); 323Science et al.M. K. Smith,
Performance on In-Class Concept Questions
Why Peer Discussion Improves Student
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plored by combining models and further em-
pirical data, but geology offers a stronger constraint
because circumstances under which sulfate can
be preserved in terrestrial sedimentary records
are uncommon.
Although various aspects of Neoproterozoic
glaciations are intensely disputed (25), our results
confirm a profound difference from Phanerozoic
ice ages. A near-global distribution of glaciated
continents during the Marinoan phase ending
~ 635 million years ago is supported by evidence
of low palaeomagnetic latitudes (26). The snow-
ball Earth model (27) predicts a progressive accu-
mulation of volcanic volatiles in the atmosphere
that are not removed by weathering until the rapid
demise of the ice age as the ice-albedo feedback
reverses. If sulfate with large negative ∆
17
O signals
derived from oxidative weathering could only be
generated in a large quantity after melting of the
“snowball”and exposure of continents, then the
diamictites above W2 had to be deposited during
final glacial retreat, a hypothesis that should prompt
a re-examination of their sedimentology. The al-
ternative “slushball”model, in which parts of the
ocean area are ice-free (28), would also permit ac-
cumulation of sulfate from prolonged oxidative
weathering in certain continental “oases”where
arid but cold conditions prevailed. This study pro-
vides an effective way to study the dynamics of
sedimentation and atmospheric-hydrosphere-
biosphere interactions during a global glaciation
and highlights the need for further stratigraph-
ically constrained ∆
17
O
SO4
data on continental
carbonate precipitates to ground-truth flux-balance
models.
References and Notes
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Acta 71, 4868 (2007).
2. d
18
Oord
17
O≡R
xsample
/R
xstandard
−1(whereR
x
=
18
O/
16
O
or
17
O/
16
O); the same dnotation applies to d
13
Cord
34
S
in this paper.
3. Reference units for stable isotope compositions: VSMOW
for sulfate d
18
O, d
17
O, and ∆
17
O; VPDB for carbonate
d
13
C and d
18
O; and Vienna Canyon Diablo Troilite for
sulfate d
34
S.
4. G. E. Claypool, W. T. Holser, I. R. Kaplan, H. Sakai, I. Zak,
Chem. Geol. 28, 199 (1980).
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Lett. 28, 283 (2001).
29. H.B. and I.J.F. designed research and led the writing of
the manuscript; H.B. performed CAS extraction and triple
oxygen isotope measurements; I.J.F secured samples
from field expeditions and conducted sedimentological,
petrographic, mineralogical and elemental studies;
P.M.W. conducted preliminary CAS extraction and
performed d
34
S
CAS
analysis; and C.S. carried out d
13
C and
d
18
O analysis of host carbonates. We thank G. Halverson
for discussion and Y. Peng for analytical assistance.
Financial and facility supports were provided by Louisiana
State University, NSF, and Chinese Academy of Science
(H.B.), Natural Environment Research Council (NERC)
standard grant (GR3/C511805/1) and NERC inductively
coupled plasma mass spectrometry facilities (I.J.F.), and
Austrian Science Funds (C.S.). The authors declare no
competing financial interests.
Supporting Online Material
www.sciencemag.org/cgi/content/full/323/5910/119/DC1
Materials and Methods
SOM Text
Figs. S1 and S2
Tables S1 and S2
References
3 September 2008; accepted 25 November 2008
10.1126/science.1165373
Why Peer Discussion Improves
Student Performance on In-Class
Concept Questions
M. K. Smith,
1
*W. B. Wood,
1
W. K. Adams,
2
C. Wieman,
2,3
J. K. Knight,
1
N. Guild,
1
T. T. Su
1
When students answer an in-class conceptual question individually using clickers, discuss
it with their neighbors, and then revote on the same question, the percentage of correct answers
typically increases. This outcome could result from gains in understanding during discussion, or
simply from peer influence of knowledgeable students on their neighbors. To distinguish between
these alternatives in an undergraduate genetics course, we followed the above exercise with a
second, similar (isomorphic) question on the same concept that students answered individually.
Our results indicate that peer discussion enhances understanding, even when none of the students
in a discussion group originally knows the correct answer.
In undergraduate science courses, conceptual
questions that students answer using personal
response systems or “clickers”are promoted
as a means to increase student learning [e.g. (1,2)],
often through peer instruction (PI) (3). Instructors
using this approach break up their lectures with
multiple-choice questions to test understanding
of the concepts being presented. When PI is used,
students are first asked to answer a question in-
dividually, and then a histogram of their re-
sponses may be displayed to the class. If there is
substantial disagreement among responses, stu-
dents are invited to discuss questions briefly with
their neighbors and then revote before the correct
answer is revealed. The instructor then displays
the new histogram and explains the reasoning
behind the correct answer. Most instructors report
that the percentage of correct answers, as well as
students’confidence in their answers, almost
always increases after peer discussion (2–4).
It is generally assumed that active engage-
ment of students during discussion with peers,
some of whom know the correct answer, leads to
increased conceptual understanding, resulting in
improved performance after PI. However, there is
an alternative explanation: that students do not in
fact learn from the discussion, but simply choose
the answer most strongly supported by neighbors
they perceive to be knowledgeable. We sought to
distinguish between these alternatives, using an
additional, similar clicker question that students
answered individually to test for gains in under-
standing. Our results indicate that peer discussion
enhances understanding, even when none of the
students in a discussion group originally knows
the correct answer.
In an undergraduate introductory genetics
course for biology majors at the University of
Colorado–Boulder (additional demographic in-
1
Department of Molecular, Cellular, and Developmental
Biology, University of Colorado, Boulder, CO 80309, USA.
2
Department of Physics, University of Colorado, Boulder,
CO 80309, USA.
3
Department of Physics, University of
British Columbia, Vancouver, BC V6T 1Z3, Canada.
*To whom correspondence should be addressed. E-mail:
michelle.k.smith@colorado.edu
2 JANUARY 2009 VOL 323 SCIENCE www.sciencemag.org122
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formation in table S1), we asked an average of
five clicker questions per 50-min class through-
out the semester and encouraged students to
discuss questions with their neighbors. Students
were given participation points for answering
clicker questions, regardless of whether their an-
swers were correct. Exam questions were similar
to the clicker questions, so that students had an
incentive to take clicker questions seriously.
Sixteen times during the semester we assessed
how much students learned from peer discus-
sion by using a paired set of similar (isomorphic)
clicker questions. Isomorphic questions have dif-
ferent “cover stories,”but require application of
the same principles or concepts for solution (5,6).
Sample isomorphic question pairs are shown in
fig. S1. In class, students were first asked to an-
swer one question of the pair individually (Q1).
Then they were invited to discuss the question
with their neighbors and revote on the same ques-
tion (Q1
ad
for “Q1 after discussion”). Finally, stu-
dents were asked to answer the second isomorphic
question, again individually (Q2). Neither the an-
swers to the two questions (Q1/Q1
ad
and Q2) nor
the histograms of student answers were revealed
until after the voting on Q2, so that there was
minimal instructor or whole-course peer influence
on the Q2 responses. The isomorphic questions
were randomly assigned as Q1/Q1
ad
or Q2 after
both questions were written. Data analysis was
limited to students who answered all three questions
of an isomorphic pair with a total of 350 students
participating in the study (7) (see supporting
online text).
Two results indicate that most students
learned from the discussion of Q1. First, using
data pooled from individual mean scores on Q1,
Q1
ad
, and Q2 for all 16 question pairs, the av-
erage percentage correct for Q2 was significantly
higher than for Q1 and Q1
ad
(Fig. 1A and Table
1). Second, of the students who answered Q1
incorrectly and Q1
ad
correctly, 77% answered Q2
correctly (Fig. 2). This result suggests that most
students who initially did not understand a con-
cept were able to apply information they learned
during the group discussion and correctly answer
an isomorphic question. In contrast, almost all
students who answered Q1 correctly, presumably
because they understood the concept initially, did
not change their votes on Q1
ad
andwentonto
answer Q2 correctly (Fig. 2).
In addition, students who answered both Q1
and Q1
ad
incorrectly still appeared to learn from
discussions with peers and answering a second
question on the same topic. Of these students,
44% answered Q2 correctly, significantly better
than expected from random guessing (Fig. 2; on
average, the questions in our 16 isomorphic pairs
had four answer choices each). This result was
unexpected because when students answered
Q2, they had not been told the correct answer to
Q1/Q1
ad
, had not seen histograms of student re-
sponses, and had not discussed Q2 with their
peers. We speculate that when this group of stu-
dents discussed Q1, they were making sense of
the information, but were unable to apply their
new knowledge until presented with a fresh ques-
tion on the same concept (Q2). There may also
be a learning benefit to considering successive
clicker questions on the same topic (8).
Although the difficulty of the question pairs
varied, as judged by the percentage of correct
answers on Q1 (see supporting online text), stu-
dents performed significantly better on Q1
ad
and
Q2 compared to Q1 for each difficulty level (Fig.
1B and Table 1). On the most difficult questions
there was another significant increase between
Q1
ad
and Q2, suggesting that there was an addi-
tional delayed benefit to the group discussions.
Fig. 1. The percentage of students
who can correctly answer a ques-
tion as individuals increases after
peer discussion of a similar (iso-
morphic) question. Q1: One ques-
tion of an isomorphic pair was
voted on individually; Q1
ad
:the
same question was voted on again
afterpeerdiscussion;Q2:the
second isomorphic question was
voted on individually. (A)Results
for all 16 question pairs were
averaged for each individual (n=
350 students), and the class aver-
ages of these scores are shown. (B)
The 16 paired questions were
grouped according to difficulty based
on the percentage of correct answers
for Q1 (five easy questions, seven
medium questions, and four difficult
questions), and performance results
were again averaged for each indi-
vidual (n= 343 students for easy,
344 for medium, and 337 for dif-
ficult) before computing the averages
shown. Error bars show the SEM.
Table 1. Mean differences between Q1, Q1
ad
, and Q2. The SEM is in parentheses.
Question category Q1
ad
–Q1* (%) Q2 −Q1* (%) Q2 −Q1
ad
* (%)
All questions 16(1) 21(1) 5(1)
Easy questions 16(1) 12(2) −4(1)
†
Medium questions 15(1) 16(2) 1(1)
†
Difficult questions 16(2) 38(2) 22(2)
*Mean values are the averages of the differences between Q1
ad
-Q1, Q2-Q1, and Q2-Q1
ad
for each student.
†
No significant
improvement between these questions.
Fig. 2. Breakdown of student responses for the pool of 16 Q1, Q1
ad
, and Q2 questions. Percentages of
the category are connected by arrows from the preceding line. Underlined entries represent students who
initially did not answer Q1 correctly but did so after group discussion; entries with an asterisk represent
students who did not answer either Q1 or Q1
ad
correctly, but nevertheless were able to correctly answer
the isomorphic question Q2. Of the 32 questions in our 16 question pairs, 7 had 5 answer choices, 5 had 4
choices, 3 had 3 choices, and 1 had 2 choices.
www.sciencemag.org SCIENCE VOL 323 2 JANUARY 2009 123
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Our results suggest that peer discussion can
be effective for understanding difficult concepts
even when no one in the group initially knows
the correct answer. In a postsemester survey (n=
98 responding), students reported an average
of three participants in their peer discussion
groups. If students who knew the answer to Q1
were randomly distributed throughout the class-
room, then on the difficult questions (Fig. 1B),
more than half of the 84 groups would have in-
cluded no one who knew the correct answer to Q1
(naïve groups). Statistical analysis (see supporting
online text) shows that some students who an-
swered Q2 correctly must have come from naïve
groups.
Student opinion supported the view that having
someone in the group who knows the correct
answer is unnecessary. On an end-of-year survey
(n= 328 responding), 47% of students disagreed
with the statement: “When I discuss clicker
questions with my neighbors, having someone
in the group who knows the correct answer is
necessary in order to make the discussion pro-
ductive.”Representative comments from these
students included the following: “Often when
talking through the questions, the group can fig-
ure out the questions without originally knowing
the answer, and the answer almost sticks better
that way because we talked through it instead of
just hearing the answer.”“Discussion is produc-
tive when people do not know the answers be-
cause you explore all the options and eliminate
the ones you know can’t be correct.”
This study supports the substantial value of
student peer discussion as an effective means of
active learning in a lecture class. Our findings are
consistent with earlier demonstrations of social
learning, including the value of discussion with
peers (9–13). The significant increases in per-
formance between Q1 and Q1
ad
confirm results
from earlier classroom studies (2–4). In addition,
we have presented new evidence showing that
these increases result primarily from student
gains in conceptual understanding rather than
simply from peer influence.
Previous explanations for the value of PI have
maintained the “transmissionist”view (14)that
during discussion, students who know the right
answer are explaining the correct reasoning to
their less knowledgeable peers, who consequently
improve their performance on the revote (3,4).
Our finding that even students in naïve groups
improve their performance after discussion sug-
gests a more constructivist explanation: that these
students are arriving at conceptual understanding
on their own, through the process of group dis-
cussion and debate.
Some instructors who use clicker questions
skip peer discussion entirely, believing that in-
structor explanation of the correct reasoning will
be more clear and accurate than an explanation
by peers, and will therefore lead to more student
learning. Although our current work does not
directly compare the benefits of instructor versus
peer explanation, research in physics has shown
that instructor explanations often fail to produce
gains in conceptual understanding (15). We have
shown that peer discussion can effectively pro-
mote such understanding. Furthermore, justifying
an explanation to a fellow student and skeptically
examining the explanation of a peer provide val-
uable opportunities for students to develop the
communicative and metacognitive skills that are
crucial components of disciplinary expertise.
References and Notes
1. D. Duncan, Clickers in the Astronomy Classroom (Pearson
Education, San Francisco, 2006).
2. J. K. Knight, W. B. Wood, Cell Biol. Educ. 4, 298 (2005).
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Hall, Saddle River, NJ, 1997).
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14. We use this term to describe the view that learning
during instruction occurs by transmission of information
from a teacher to a learner.
15. D. Hestenes, Phys. Teach. 30, 141 (1992).
16. M.K.S., W.K.A. and J.K.K. were supported by the
University of Colorado Science Education Initiative.
Supporting Online Material
www.sciencemag.org/cgi/content/full/323/5910/122/DC1
SOM Text
Fig. S1
Tables S1 to S3
15 September 2008; accepted 10 November 2008
10.1126/science.1165919
Regulation of Neuronal
Survival Factor MEF2D by
Chaperone-Mediated Autophagy
Qian Yang,
1
Hua She,
1
Marla Gearing,
2
Emanuela Colla,
3
Michael Lee,
3
John J. Shacka,
4
Zixu Mao
1,2
*
Chaperone-mediated autophagy controls the degradation of selective cytosolic proteins
and may protect neurons against degeneration. In a neuronal cell line, we found that
chaperone-mediated autophagy regulated the activity of myocyte enhancer factor 2D (MEF2D),
a transcription factor required for neuronal survival. MEF2D was observed to continuously shuttle
to the cytoplasm, interact with the chaperone Hsc70, and undergo degradation. Inhibition of
chaperone-mediated autophagy caused accumulation of inactive MEF2D in the cytoplasm.
MEF2D levels were increased in the brains of a-synuclein transgenic mice and patients with
Parkinson’s disease. Wild-type a-synuclein and a Parkinson’s disease–associated mutant disrupted
the MEF2D-Hsc70 binding and led to neuronal death. Thus, chaperone-mediated autophagy
modulates the neuronal survival machinery, and dysregulation of this pathway is associated with
Parkinson’s disease.
In neurodegenerative diseases, certain popula-
tions of adult neurons are gradually lost because
of toxic stress. The four myocyte enhancer fac-
tor 2 (MEF2) transcription factors, MEF2A to
MEF2D, have been shown to play an important
role in the survival of several types of neurons, and
a genetic polymorphism of the MEF2A gene has
been linked to the risk of late onset of Alzheimer’s
disease (1–3). In cellular models, inhibition of
MEF2s contributes to neuronal death. Enhancing
MEF2 activity protects neurons from death in
vitro and in the substantia nigra pars compacta in a
mouse model of Parkinson’s disease (PD) (4). Neu-
rotoxic insults cause MEF2 degradation in part by a
caspase-dependent mechanism (5), but how MEF2
is regulated under basal conditions without overt
toxicity is unknown. Autophagy refers to the deg-
radation of intracellular components by lysosomes.
Relative to macro- and microautophagy, chaperone-
mediated autophagy (CMA) selectively degrades
cytosolic proteins (6). This process involves bind-
ingofheatshockproteinHsc70tosubstratepro-
teins via a KFERQ-like motif and their subsequent
targeting to lysosomes via the lysosomal membrane
receptor Lamp2a. Dysregulation of autophagy
plays a role in neurodegeneration (7–9). However,
the direct mechanism by which CMA modulates
neuronal survival or death is unclear.
1
Department of Pharmacology, Emory University School of
Medicine, Atlanta, GA 30322, USA.
2
Department of Neurology,
Emory University School of Medicine, Atlanta, GA 30322, USA.
3
Department of Pathology, Johns Hopkins University School of
Medicine, Baltimore, MD 21205, USA.
4
Department of
Pathology, Division of Neuropathology, University of Alabama
at Birmingham, Birmingham, AL 35294, USA.
*To whom correspondence should be addressed. E-mail:
zmao@pharm.emory.edu
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