Is project-based learning effective among kindergarten and elementary students? A systematic review

Abstract

Project-based learning (PjBL) is becoming widespread in many schools. However, the evidence of its effectiveness in the classroom is still limited, especially in basic education. The aim of the present study was to perform a systematic review of the empirical evidence assessing the impact of PjBL on academic achievement of kindergarten and elementary students. We also examined the quality of studies, their compliance with basic prerequisites for a successful result, and their fidelity towards the key elements of PBL intervention. For this objective, we conducted a literature search in January 2020. The inclusion criteria for the review required that studies followed a pre-post design with control group and measured quantitatively the impact of PBL on content knowledge of students. The final sample included eleven articles comprising data from 722 students. The studies yielded inconclusive results, had important methodological flaws, and reported insufficient or no information about important aspects of the materials, procedure and key requirements from students and instructors to guarantee the success of PjBL. Educational implications of these results are discussed.

Full text

RESEARCH ARTICLE
Is project-based learning effective among
kindergarten and elementary students? A
systematic review
Marta FerreroID
1
, Miguel A. Vadillo
2
*, Samuel P. Leo
´n
3
*
1Departamento de Investigacio
´n y Psicologı
´a de la Educacio
´n, Universidad Complutense de Madrid,
Madrid, Spain, 2Departamento de Psicologı
´a Ba
´sica, Universidad Auto
´noma de Madrid, Madrid, Spain,
3Departamento de Pedagogı
´a, Universidad de Jae
´n, Jae
´n, Spain
*miguel.vadillo@uam.es (MAV); sparra@ujaen.es (SPL)
Abstract
Project-based learning (PjBL) is becoming widespread in many schools. However, the evi-
dence of its effectiveness in the classroom is still limited, especially in basic education. The
aim of the present study was to perform a systematic review of the empirical evidence
assessing the impact of PjBL on academic achievement of kindergarten and elementary stu-
dents. We also examined the quality of studies, their compliance with basic prerequisites for
a successful result, and their fidelity towards the key elements of PBL intervention. For this
objective, we conducted a literature search in January 2020. The inclusion criteria for the
review required that studies followed a pre-post design with control group and measured
quantitatively the impact of PBL on content knowledge of students. The final sample
included eleven articles comprising data from 722 students. The studies yielded inconclu-
sive results, had important methodological flaws, and reported insufficient or no information
about important aspects of the materials, procedure and key requirements from students
and instructors to guarantee the success of PjBL. Educational implications of these results
are discussed.
Introduction
Over the last decade, numerous institutions have addressed the skills and dispositions that are
expected to be vital for schooling in 21st century. Some of these skills are critical thinking,
communication, collaboration, or creativity [1,2]. According to many experts, although the
prevailing methods of direct instruction and recitation may be effective for the acquisition of
factual knowledge, these skills demand new pedagogical approaches [3]. Within this context,
project-based learning (PjBL) and problem-based learning (PBL) have emerged as valuable
inquiry approaches to achieve the so-called skills for the 21st century [4].
PjBL and PBL are usually described as active, student-centred methods of instruction that
encourage students to work in collaborative groups on real-world questions or challenges to
promote the acquisition of higher-order thinking skills, while teachers act as facilitators of
learning [4–14]. Despite these common characteristics, PjBL and PBL also present some
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OPEN ACCESS
Citation: Ferrero M, Vadillo MA, Leo
´n SP (2021) Is
project-based learning effective among
kindergarten and elementary students? A
systematic review. PLoS ONE 16(4): e0249627.
https://doi.org/10.1371/journal.pone.0249627
Editor: Mingming Zhou, University of Macau,
MACAO
Received: July 11, 2020
Accepted: March 23, 2021
Published: April 2, 2021
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0249627
Copyright: ©2021 Ferrero et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
reported within the paper. As the present
systematic review does not include a quantitative
meta-analysis, there are no additional datafiles,

noticeable differences. For instance, in PjBL learners are expected to follow correct procedures
towards a desired end-product or presentation during which they are likely to encounter dif-
ferent problems [7,9,14], while in PBL the emphasis is on the role of the students to define the
problem and develop a solution [9,15,16]. In addition, while in PBL the solution to the prob-
lem is merely suggested, in PjBL it must be executed [7]. Finally, PjBL occurs over an extended
time period, while PBL normally lasts a few days [5]. In practice, given the usual difficulties in
distinguishing one from the other or in defining their key features [5,9,14], both terms are
often employed interchangeably among researchers [4,14] and teachers [17]. Since both
approaches are closely related and share a central end, throughout this review we will use the
term PjBL to refer to both of them.
PjBL originated in an architecture school in Rome in the 16th century [18]. Forced by orga-
nizational and curricular constraints, lectures were moved to weekends and, to minimize the
potential lack of motivation among students, teachers decided to use this approach. Later on,
dissatisfaction with standard methods in medical education led a large number of medical
schools to adopt PjBL [6], which progressively extended to different undergraduate studies
[10,15,19]. The main reasons for adopting this approach were student disenchantment and
boredom caused by the vast amount of information they had to learn with presumably little
impact on daily practice [6]. In general, the quantitative reviews performed in medical schools
show that the traditional approach to learning in the classroom outperforms PjBL in the acqui-
sition of basic science knowledge, while, conversely, PjBL is superior to the traditional
approach when it comes to learning clinical problem solving, that is, application of knowledge
[8,20–23] and ability to link concepts [19,24]. More generally, different studies conducted with
undergraduate students have shown that PjBL can help students improve academic achieve-
ment [25] and build flexible knowledge [10].
In spite of the promising results of PjBL, some authors have drawn attention to the existing
gaps in our knowledge about the conditions under which PjBL can be more beneficial than
other approaches [21]. Similarly, researchers have outlined the importance of considering
some prerequisites necessary for students and teachers to be successful in higher education
when using PjBL. In the case of students, these requisites include the previous acquisition of
basic content knowledge about the target problem or project and competence in some learning
strategies and skills (i.e., the ability to communicate ideas effectively). For teachers, the requi-
sites include, for instance, proficiency in appropriate teaching strategies and tools (i.e., the pro-
vision of adequate scaffolding). If these prerequisites are not met, students might not benefit
from PjBL and teachers might not be able to apply it with any guarantee of success [7]. Finally,
due to the various ways in which PjBL has been implemented in the classroom, it is important
to pay attention to the fidelity with which its main principles are applied when evaluating its
impact on learning. Ideally, an intervention faithful to the PjBL approach should include all its
essential components as defined in the literature. Otherwise, there is a risk of attributing the
(positive or negative) effects of an intervention to PjBL when, in fact, the intervention does not
meet the definition of PjBL. As mentioned above, some of the central elements to PjBL are the
need of a problem to drive the activities and a final artifact or product; the use of group work
methodology; the empowerment of students; the provision of guidance and resources by
teachers; and the adoption of evaluation tools adapted to PjBL characteristics (i.e., notebook
entries or portfolio).
The effectiveness of PjBL has also been tested at the secondary school level, although to a
lesser extent than in medical schools and undergraduate studies. As in the case of undergradu-
ate settings, this approach has been shown to improve the academic achievement of secondary
school students in different subjects, such as economics [26,27], history [28], or STEM (Science,
Technology, Engineering, and Mathematics) [29–31]; for a review, [5,25,32]. In spite of these
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beyond the information reported in the results
section.
Funding: Funded by Agencia Estatal de
Investigacio
´n with grant number PSI2017-85159-P
(MAV), Comunidad de Madrid (ES) with grant
number 2016-T1/SOC-1395 (MAV) and
Comunidad de Madrid (ES) with grant number
2020-5A/SOC-19723 (MAV). Funders did not play
any role in any phase of the study.
Competing interests: The authors have declared
that no competing interests exist.

promising results, some researchers have warned of the limited number of scientific studies on
PjBL instruction in high school and emphasize the need for more and better research before
strong claims can be made about the potential benefits of this approach [5,25,33]. Furthermore,
most of the studies conducted to date followed quasi-experimental designs, so the existing evi-
dence on the impact of PjBL in secondary school level appears to be weak [34].
At present, a growing number of kindergarten and primary schools are introducing PjBL in
their classrooms. Even more, in countries like Spain, the educational authorities of some
regions have made the inclusion of PjBL in classroom programmes mandatory [35]. Consider-
ing the good results obtained in higher levels, it is reasonable to expect that PjBL would also
contribute to promoting the learning of kindergarten and primary students. Nevertheless, due
to the considerable differences between senior and novice learners [36], this assumption
deserves further analysis. Unlike the cases mentioned above, there is still no systematic review
on the efficacy of PjBL exclusively focused on these basic levels of education. To our knowl-
edge, there are two non-systematic reviews and one meta-analysis that have addressed the
effectiveness of PjBL in different levels, including to some extent kindergarten and primary
education. The first one focuses on the effect of PjBL in students from kindergarten to K-12. It
includes both quantitative and qualitative studies [11]. The second one is an overview of the
effectiveness of PjBL from preschool to higher education and pre-service teacher training [12].
And the third one analyses quantitatively the impact of PjBL on academic achievement in
comparison with traditional teaching from third grade elementary school to senior college stu-
dents and explores what study features might moderate this effect [25]. Overall, these studies
conclude that PjBL is an effective means of teaching content information. However, in all
cases, important pieces of information are missing from the studies analysed. For instance,
none of the reviews assess the level of student and instructor compliance with the basic
requirements of PjBL. Similarly, the fidelity of interventions to the main principles of PjBL is
not analysed. Finally, only one of the studies [11] analyses the information related to the qual-
ity of the primary studies. Considering that the authors of these reviews have highlighted the
need of better and more detailed research, it seems advisable to report and discuss this type of
information more thoroughly. Without this information, it is difficult, if not impossible, to
draw firm conclusions about the effectiveness of PjBL for kindergarten and elementary school
students.
The main objective of the present study was to perform a systematic review on the effect of
PjBL on the acquisition of content knowledge in kindergarten and primary students, including
as much relevant information as possible on methodological and conceptual aspects. Specifi-
cally, we examined the quality of existing studies, their compliance with basic prerequisites for
a successful PjBL intervention, and the fidelity to the interventions in light of the key elements
of PjBL, as reported in the literature.
Method
Search procedures
The present systematic review follows the PRISMA recommendations. On January 23th 2020
the first author (MF) performed an electronic search on the Web of Science,PsycInfo, and
ERIC entering the terms “(project based OR problem based) AND (learning OR intervention
OR approach OR instruction)” into the Topic field. The search was limited to (a) articles in
English, (b) published between 1900 and 2020, (c) with categories restricted to “education/
educational research” and “psychology”. Unpublished dissertations, reviews and meta-analyses
were excluded at this stage. After removing 523 duplicates this initial search yielded a sample
of 34,246 studies.
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The titles and abstracts of these studies were screened by MF using the inclusion criteria
c1-c5 explained below. This resulted in the exclusion of 32,208 studies that did not meet the
inclusion criteria. MF and SPL independently read the full text of the remaining 38 studies to
verify that they fulfilled criteria c1-c5. Among the initial set of 38 articles assessed for eligibility,
nine articles met the inclusion criteria. Thereupon, we performed descendancy searches of
articles citing or cited by these nine papers to identify additional studies. The titles and
abstracts of the second search were screened by MF and this resulted in 16 full-text articles
that were also independently read by MF and SPL. No additional study was selected from this
set. Finally, on request of an anonymous reviewer, we added two extra studies included in a
meta-analysis. Therefore, the final sample of articles reviewed for inclusion comprised eleven
articles (see Table 1) [37–47]. Fig 1 shows a PRISMA flowchart summarizing the literature
search process. Across all the full-text articles read for inclusion, the initial inter-rater agree-
ment was 98.31%. Disagreements were resolved by discussion and consensus between the two
researchers until there was 100% agreement.
Selection criteria
The studies were only included if they met the following criteria: c1) the aim was to evaluate
the effect of PjBL on content knowledge; c2) they followed a pre-post design with control
group; c3) the target sample comprised students from kindergarten to grade 6; c4) they were
written in English; and c5) they were peer-reviewed. Therefore, narrative and systematic
reviews, doctoral dissertations, posters, registered study protocols, commentaries, books and
book chapters, essays, and other theoretical reports were excluded from the review.
Data extraction and coding
The eleven studies that met the inclusion criteria were independently examined in depth and
coded by MF and SPL. They recorded information related to general aspects (authors, year of
publication, and journal), participants (country of origin, sample size, age, educational level,
and school type), method (design, duration, dependent variable, and measuring tools), and the
main results obtained by each study.
In order to overcome important shortcomings of the reviews mentioned above, we used the
quality scale developed by [48] with just one modification (see below). Very briefly, the origi-
nal 17-item scale includes information related to the quality of various methodological aspects
of an empirical research such as randomisation, blinding, replicability, or test validity (see Fig
2). Each item could be assigned three values: positive, negative, and unknown. For each study,
MF and SPL independently assigned a value to each item, reaching an initial agreement of
98.30%. Disagreements were resolved through discussion until 100% consensus was reached.
Fig 2 shows the values assigned to each item and study.
The quality scale used in [48] was originally created to assess educational interventions inspired
by the multiple intelligences theory. Unlike research in that field, the PjBL literature offers a wealth
of information on the basic prerequisites that both students and instructors should meet for PjBL
to be successful, as well as on the key principles that characterize this approach. Therefore, for this
study, we removed Item 6 from the original scale (referring to intervention fidelity) and replaced it
by a full new scale intended to analyze both the compliance of teachers and students with the basic
prerequisites of PjBL and the fidelity of the intervention to the principles underlying this method.
This new scale consists of 30 items divided in two parts. Part A refers to the prerequisites and Part
B refers to intervention fidelity. The 14 items in Part A are grouped into three categories. Items a1
to a6 belong to the category “Previous training of students in group work”, Item a7 to “Measure-
ment of prior knowledge of students”, and Items a8 to a14 to “Teacher training in PjBL”. The 12
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Table 1. Articles that met inclusion and quality criteria.
Authors,
year
Country Sample
size (E/
N)
Age
(mean)
Sample
type
Educational
level
School
type
Instructor Duration Dependent
variable
Tests to measure
DV
Results
Alacapinar,
2008 [37]
Turkey 42 (21,
21)
(11.4
years)
normal
population
5th grade n.s. n.s. n.s. Cognitive
domain
n.s. PBL group
outperformed
significantly
control group.
Aral et al.,
2010 [38]
Turkey 28 (14,
14)
6 year normal
population
Preschool
education
n.s. n.s. 12 weeks
(1 day per
week)
Children’s
conceptual
development
and school
readiness
composite
Bracken Basic
Concept Scale-
Revised
No differences
between PBL and
control group.
Aslan, 2013
[39]
Turkey 47 (24,
23)
6 year normal
population
Preschool
education
public
school
Teacher 12 weeks
(3 days
per week)
Categorization
skills
A categorization
test
PBL group
outperformed
significantly
control group.
C¸ akici et al.,
2013 [46]
Turkey 44 (22,
22)
n.s. normal
population
5th grade public
school
Teacher
and
researcher
5 weeks Sciences
knowledge
The Light and
Sound
Achievement
Test
PBL group
outperformed
significantly
control group.
Can et al.,
2017 [40]
Turkey 26 (17,
9)
6 year normal
population
Preschool
education
n.s. Teacher 32 weeks Scientific process
skills and
conceptions
Preschool
Scientific Process
Skills Scale
No comparison
reported between
PBL and control
group.
Gu¨ ltekin,
2005 [41]
Turkey 40 (20,
20)
n.s. normal
population
5th grade n.s. n.s. 3 weeks (6
hours per
week)
Achievement in
social studies
An achievement
test
No quantitative
data reported.
Hastie et al.,
2017 [42]
EEUU 185
(109,
76)
(10.6
years)
normal
population
5th grade rural
school
Teacher
and
researcher
9 week Fitness
knowledge
Fitness
Knowledge Test
PBL group
outperformed
significantly
control group.
Karac¸alli
et al., 2014
[43]
Turkey 143 (73,
70)
9–11
years
normal
population
4th grade n.s. Teacher
and
researcher
4 weeks Sciences
knowledge
Electricity in Our
Life
Achievement
Test (ELACH),
Science Course
Attitude Scale
(ELATT)
PBL group
outperformed
significantly
control group.
Kucharski
et al., 2005
[47]
EEUU 61 (30,
31)
n.s. normal
population
1st, 3th and
4th grade
n.s. Teacher n.s. Sciences
knowledge
Terra Nova Scale PBL group
outperformed
significantly
control group
(except in 4th
grade).
Lin, 2015
[44]
Taiwan 56 (28,
28)
11
years
normal
population
5th grade public
school
Teacher 12 weeks
(40 min
per week)
Vocabulary
knowledge
Vocabulary
knowledge test
No differences
between PBL and
control group.
Zumbach
et al., 2004
[45]
Germany 50 (24,
26)
(10.1
years)
normal
population
4th grade n.s. Teacher
and
computer
n.s. Forest animals
knowledge
A konowledge
test
No differences
between PBL and
control group in
the short-term
but yes in the
long- term for
PBL group.
Note: DV: Dependent variable. (E) Experimental Group. (C) Control Group. n.s.: Not specified.
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items of Part B are grouped into seven categories. Item b1 belongs to the category “Realism of the
matter raised”, Item b2 to “Existence or not of a final product”, Item b3 to “Inclusion or not of
group work”, Items b4 to b7 to “Scaffolding by the teacher during learning”, Item b8 to “Auton-
omy granted to students when making decisions about the project”, Items b9 to b12 to “Correct
evaluation tools employed”, and Items b13 to b16 to “Explicit practice of metacognitive skills”.
The categories which conform the scale were elaborated based on the principles suggested by ref-
erence review works in this field [5,7,9,11,14]. For the sake of consistency, each category in the
scale must have been mentioned by at least two of these reference sources. As in the quality scale
mentioned above, each item could obtain one of three values. MF and SPL independently scanned
all the studies and assigned a value to each item, reaching an initial agreement of 99.63%. Disagree-
ments were resolved by discussion and consensus between the two researchers until there was
100% agreement. Fig 3 shows a detailed description of the values assigned to each item.
Results
Description of the studies
Table 1 provides a detailed summary of the eleven studies included in this review. Overall,
many of the coded elements showed substantial heterogeneity, such as sample size, age of par-
ticipants, duration of the interventions, or reported outcomes. Specifically, the total sample
Fig 1. PRISMA flowchart. Flow of information through the different phases completed in the systematic review.
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consisted of 722 participants, aged between 6 and 11 years. Among them, 101 were kindergar-
ten students and 621 were first- to sixth-grade students. The interventions lasted between 4
and 32 weeks. Some of the subjects covered were science, mathematics, or English.
As can be seen in Table 1, most of the studies included in this review reported positive effects
of PjBL on academic achievement. More precisely, six studies showed significant improvement
of students trained through PjBL compared to students trained with other methods; three stud-
ies obtained improvements through both methods (without making comparisons between the
experimental and control groups); one study found no significant difference between PjBL and
other types of training; and the remaining study reported no quantitative data.
Quality scale
Fig 2 shows the results of the qualitative assessment of the eleven studies included in the review.
Across all items, 28.98% were rated as positive, 41.48% as negative, and 29.55% as unknown.
Fig 2. Scale of quality and values assigned to each item. Summary of the items which comprise the scale of quality and values assigned to
each of them in each study.
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Fig 3. Scale of prerequisites and intervention fidelity. Summary of the prerequisites necessary for students and teachers for a successful PjBL
adoption and intervention fidelity criteria in light of the key elements of the method.
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Most of the studies followed a quasi-experimental design (Items 2 and 3) and did not include an
active control group (Item 10). None of the studies guaranteed blinding of participants, instruc-
tors, and evaluation or, alternatively, did not report any information on this matter (Items 4 to
6). Similarly, most of the studies failed to provide enough information to replicate the interven-
tion or the dependent variable (Items 11 and 12), and no study informed about the validity of
the latter (Item 14). None of the studies had been preregistered or made the data publicly avail-
able on the Internet (Items 1 and 16). In contrast, most of the studies confirmed the similarity
of the experimental and control groups in terms of socio-economic characteristics (Item 7).
Likewise, most of the studies reported the analysis of pre-test scores in experimental and control
groups (Item 8) and analyzed the differences between them (Item 15).
Prerequisites and intervention fidelity scale
Fig 3 shows a summary of the information related to the prerequisites and intervention fidelity
of the studies. As can be seen, overall 20.61% of the items obtained positive values, 0.61%
obtained negative values, and 78.79% were labeled as unknown. Most studies offered little or
no information to assess the items related to compliance with prerequisites (Part A). Only one
study reported specific information about the training of students in group work (Items a1 to
a6) and it focused exclusively on the ability to discuss ideas (Item a1). Similarly, just two stud-
ies informed about the training of teachers in PjBL (Item a8 to a14) but none of them provided
any information about the content of this training and, consequently, they were coded as
“unknown”. Finally, information related to prior knowledge of students before starting the
project was reported in three studies (Item a7).
In comparison, the studies reported more information about the intervention fidelity (Part
B). Overall, 51.82% of the items obtained positive values, 1.82% obtained negative values, and
46.36% were labeled as unknown. Items b1-b3, coding for the realism of the problem, the exis-
tence of a final product, and the inclusion of group work were relatively well reported and
received positive scores. Within the items focused on scaffolding, Items b4 and b6 were met by
more than half of the studies, while Item b5 was only reported by two studies and Item b7 was
not addressed in any study. Item b8, related to the autonomy provided to students, was well
reported by more than half of the studies, but, importantly, two of them received negative
scores. The rest of the items related to the appropriateness of evaluation tools (Items b9-b12)
and to the explicit practice of meta-cognitive skills (Items b13-b16) were generally reported
with insufficient detail, except for Item b16, where 6 studies obtained positive scores. Overall,
the information about intervention fidelity was often reported too vaguely and had to be
inferred indirectly from information scattered throughout the papers. For example, in the
study of Alacapinar (2008) [37], Item b13, related to planning skills, was inferred on the basis
of the following statement: "[Students] learned by experience how important it is to plan work
and accomplish it in a given time" (p. 28).
Discussion
PjBL is a student-centered methodology that promotes the acquisition of higher-order thinking
skills thought the solution of real problems in collaborative groups and with limited guidance of
the teacher [6,9,10]. Although this approach has become the cornerstone of innovative move-
ments in many schools [49,50], the evidence supporting its effectiveness in the classroom is still
scarce [5,51]. The objective of the present review was to assess the available evidence about the
impact of PjBL on the acquisition of content knowledge by kindergarten and primary students.
The articles aimed at examining the impact of PjBL in kindergarten and primary students
were scarce and, overall, yielded mixed results. Specifically, seven of the 11 studies included in
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this review obtained positive results regarding the impact of PjBL in academic achievement of
students immediately after the intervention [36,38,41,42,46,47] or in the long term [44].
Among the rest, two studies did not find significant differences between the experimental and
control groups [37,43], one study did not report any quantitative data [40], and another one
offered no comparison between the experimental and control groups [39]. In addition, the
studies showed considerable heterogeneity in terms of participants’ age (from preschoolers to
11 years old students), duration of the intervention (ranging from 3 to 32 weeks), and mea-
sured outcomes (e.g., categorization skills or English knowledge). This hinders the generaliza-
tion of the results to the entire school population.
Along with the mixed results obtained, an in-depth analysis of the studies showed impor-
tant shortcomings that deserve more attention in future research. Firstly, there is room for
improvement in the methodological quality of the studies. For instance, none of them followed
an experimental design and only two included an active control group. These deficiencies
make it hard to draw meaningful conclusions from the results. In fact, if all of these results had
been collated in a quantitative meta-analysis without a proper analysis of their quality, most
likely the conclusions would have been deceivingly positive. Without more and better evi-
dence, it is difficult to assess whether PjBL is effective for kindergarten and elementary school
students.
Secondly, information concerning important aspects of the materials and procedure was
usually not reported or, when reported, revealed suboptimal methods, compromising the rep-
licability of the studies. For example, many authors did not provide information about the
tests used to measure the outcomes, the specific activities performed, or the intervention mate-
rials. Even if PjBL was successful, in all these cases it would be impossible to bring the interven-
tion proposals to the classroom. This becomes more concerning if we consider the lack of a
universally accepted model of PjBL [14]. In the same vein, none of the studies granted access
to the data, which means that the reproducibility of the results cannot be verified by indepen-
dent researchers.
Thirdly, few studies reported sufficient information to ensure that the interventions met
the necessary requirements from students and instructors to guarantee the success of PjBL.
For instance, only three studies measured prior knowledge of students before the interven-
tion, only one offered information about students’ training in group work, and none
described the training of instructors in PjBL, had it existed. The importance of these ele-
ments is often highlighted in the literature [5,7,9,15,52]. Precisely, a recent literature
review by [33] highlighted two of them as essential for the success of PjBL: effective group
work of pupils and support to teachers through regular networking and professional devel-
opment opportunities. Given the lack of detailed information on these aspects in the stud-
ies included in this review, it is impossible to weight the contribution of these factors to the
final results.
Finally, regarding intervention fidelity, although several of the key components of PjBL
were well covered in the majority of studies (e.g., the use of real word-problems, the elabora-
tion of a final product, or collaborative work), others were broadly neglected (e.g., the amount
of guidance provided to students during the intervention, the evaluation tools used by teach-
ers, or the training on metacognitive skills). As in the case of the prerequisites mentioned
above, a considerable volume of research has stressed the importance of considering these ele-
ments in PjBL, including the monitoring of students [10,15,33,53] or the employment of ade-
quate assessment tools to measure the progress of pupils [11,33]. These information gaps
impede to determine what is decisive in this kind of intervention to be effective and, at the
same time, hamper the distinction between PjBL and other educational interventions. This
concern has been raised in previous reviews [21,54].
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Classroom implications and future research
PjBL provides highly desirable benefits for students, such as the creation of independent, self-
regulated learners [12,55,56], the promotion of engagement towards learning [9,50,57–60], or
the fostering of meaningful learning [50,61]. However, more and better evidence is needed
about how and when PjBL is most suitable. For the purpose of this review, it is relevant to con-
sider that the majority of studies assessing the impact of PjBL on learning have been aimed at
higher education students [9]. But what is effective in a secondary or a postsecondary setting
may not transfer directly to kindergarten and primary students [25]. It would be convenient to
reflect on the suitability of this approach for younger students. Specifically, it should not be
assumed that novice learners possess the advanced self-regulation skills, prior knowledge, or
group work skills (for example, the abilities needed to discuss ideas, consider alternatives, or
compare different points of view) necessary for PjBL [7,9,36]. Hence, any attempt to translate
the main results and conclusions of this literature to kindergarten and primary students should
be properly monitored. Apart from the educational stage, little is known about how different
learning profiles might make PjBL more or less effective, as in the case of learners with differ-
ent educational background or those with learning disabilities [9,14,25]. The studies included
in this review do not contribute to this question, since the population of all them is composed
by students without special needs.
Last but not least, we think that future research in this domain should try to overcome the
shortcomings we encountered in conducting this review. This includes the lack of active con-
trol groups; the lack of randomly assigned participants; inappropriate blinding of participants,
instructors, and evaluators; un-validated measures for the learning outcomes; or the lack of
detailed information to replicate the study (e.g., activities conducted, approximate duration of
each session, or evaluation tools used). Future research should also address the impact of some
basic prerequisites by students (e.g., group work) and teachers (e.g., evaluation tools) on PjBL
intervention. Similarly, it would be advisable that researches provide detailed information
about the fidelity of the intervention to key features of PjBL [11,14,33]. These important short-
comings should also be taken into consideration in interpreting or applying already published
interventions.
The academic success of many students, especially those with learning difficulties, depends
largely on the use of methods that have proven to be consistently effective [62]. For this to be
possible, the incorporation of research findings into decision making process, along with the
tacit knowledge, values, and thoughts of educators, becomes indispensable. The adoption of
this approach, known as research-informed practice, is a daunting challenge and involves
many different actors and stakeholders [63,64]. In view of the above, researchers can surely
contribute to this aim by providing more and better evidence on the conditions under which
PjBL is effective.
Limitations of the present review
The main limitation of the present review is the scarce number of studies found. From the
almost 40 full-text articles initially screened for eligibility, just nine met the selection criteria.
Similarly, from the 30 studies contained in the meta-analysis suggested by one anonymous
reviewer, just two were finally added. In light of this, it is difficult to draw firm conclusions
about the effectiveness of PjBL for kindergarten and elementary school students, beyond
highlighting shortcomings that should be addressed in future research. A wider search for
studies, perhaps not limited to peer-reviewed articles (such as papers presented at confer-
ences), might have yielded more results, although this option would most likely diminish the
average quality of the final sample of studies. Besides, given that PjBL is particularly
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recommended for the development of domain-general skills [5,9], it would have been interest-
ing to test the impact of PjBL not only on academic achievement but also on the development
of higher-order skills, such as problem solving, critical thinking, deep understanding, or self-
evaluation of students.
Supporting information
S1 checklist.
(DOC)
Author Contributions
Conceptualization: Marta Ferrero, Samuel P. Leo
´n.
Formal analysis: Marta Ferrero, Samuel P. Leo
´n.
Methodology: Marta Ferrero, Miguel A. Vadillo, Samuel P. Leo
´n.
Writing – original draft: Marta Ferrero, Samuel P. Leo
´n.
Writing – review & editing: Marta Ferrero, Miguel A. Vadillo, Samuel P. Leo
´n.
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