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Code Process Metrics in University Programming Education


Abstract and Figures

Code process metrics have been widely analyzed within large scale projects in the software industry. Since they reveal much about how programmers collaborate on tasks, they could also provide insights in the programming and software engineering education at universities. Thus, we investigate two courses taught at the University of Bamberg, Germany to gain insights into the success factors of student groups. However, a correlation analysis of eight metrics with the students' scores revealed only weak correlations. In a detailed analysis, we examine the trends in the data per assignment and interpret this using our knowledge of code process metrics and the courses. We conclude that the analyzed programming projects were not suitable for code process metrics to manifest themselves because of their scope and students' focus on the implementation of functionality rather than following good software engineering practices. Nevertheless, we can give practical advice on the interpretation of code process metrics of student projects and suggest analyzing projects of larger scope.
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Code Process Metrics in University Programming
Linus W. Dietz, Robin Lichtenthäler, Adam Tornhill, and Simon Harrer§
Department of Informatics, Technical University of Munich, Germany,
Distributed Systems Group, University of Bamberg, Germany,
Empear, Sweden,
§innoQ Deutschland GmbH, Germany,
Abstract—Code process metrics have been widely analyzed
within large scale projects in the software industry. Since they
reveal much about how programmers collaborate on tasks, they
could also provide insights in the programming and software
engineering education at universities. Thus, we investigate two
courses taught at the University of Bamberg, Germany to gain
insights into the success factors of student groups. However, a
correlation analysis of eight metrics with the students’ scores
revealed only weak correlations. In a detailed analysis, we
examine the trends in the data per assignment and interpret this
using our knowledge of code process metrics and the courses.
We conclude that the analyzed programming projects were not
suitable for code process metrics to manifest themselves because
of their scope and students’ focus on the implementation of
functionality rather than following good software engineering
practices. Nevertheless, we can give practical advice on the
interpretation of code process metrics of student projects and
suggest analyzing projects of larger scope.
When teaching programming or practical software engineer-
ing courses, lecturers often give students advice on how to
manage their group work to be successful. Such advice could
be to start early so students don’t miss the deadline, or to
split up the tasks so everybody learns something. Intuitively,
such practices deem appropriate, but do they actually lead
to more successful group work? To answer this, objective
metrics are needed as evidence. Code process metrics capture
the development progress [1], as opposed to looking at the
outcome of using static code analysis [2]. Since they have
been successfully used in the software industry [3], they
might be useful in programming education to give advice on
how to organize the development process of student projects.
As a first step towards applying code process metrics in
programming education, we want to assess their explanatory
power considering students’ success. We mine and analyze
Git repositories of two programming courses to answer our
research question: “How meaningful are code process metrics
for assessing the quality of student programming assignments?”
By this, we hope to gain insights and provide recommendations
to lecturers teaching such courses.
The subject of analysis are two practical programming
courses for undergraduate computer science students at the
University of Bamberg: ‘Advanced Java Programming’ (AJP)
covering XML serialization, testing and GUIs, and ‘Introduc-
tion to Parallel and Distributed Programming’ (PKS) covering
systems communicating through shared memory and message
passing on the Java Virtual Machine. Students typically take
AJP in their third semester and PKS in their fifth.
Course # Technologies
AJP 1 IO and Exceptions
2 XML mapping with JAXB and a CLI-based UI
3 JUnit Tests and JavaDoc documentation
4 JavaFX GUI with MVC
PKS 1 Mutexes, Semaphores, BlockingQueue
2 Executor, ForkJoin, and Java Streams
3 Client/server with TCP
4 Actor model with Akka
The courses follow a similar didactic concept that has
constantly been evolved since 2011 [2]. During the semester,
the students submit four two-week assignments (see Table I)
solved by groups of three. These assignments require the
application of the previously introduced programming concepts
and technologies from the lectures to solve realistic problems,
such as implementing a reference manager or an issue tracker.
For each assignment, the groups get a project template with
a few predefined interfaces. We provide a Git repository for
each group to work with and to submit their solutions. Since
undergraduates in their third term are usually not proficient
with version control systems, we also hold a Git tutorial at
the beginning of the course, covering how to commit, push,
merge, resolve conflicts, and come up with good commit
messages. More advanced topics like working with feature
branches or structured commit messages are not in scope of
this introduction.
We grade each assignment in form of a detailed textual code
review and a score between
points. The main part
of that score accounts for functional correctness, which we
check with the help of unit tests. However, we also evaluate
the code quality, determined by a thorough code review. To
avoid bias from one lecturer, we established a peer-review by
the other lecturer. By this, the score should be an objective
indicator for the quality of the solution. Over the years, we
23ISEE 2019: 2nd Workshop on Innovative Software Engineering Education @ SE19, Stuttgart, Germany
have built a knowledge base of typical code quality issues,
recently culminating into the book Java by Comparison [4],
which we use to refer to issues in the textual code review.
A. Data Set
The data analyzed in this paper are the Git repositories
of one iteration of AJP (
groups) and PKS (
in the academic year of 2016. This results in a total of
submissions. All groups submitted four assignments and
no group scored less than 10 points in any assignment. An
assignment solution consists of all the commits related to the
assignment. Each commit includes its message, the changes
made, a time stamp, and the author. Each submission had
at most three authors, however, this number was sometimes
reduced to two, in case a student dropped out of the course.
Because of their limited experience with Git, the students
worked solely on the master branch. Furthermore, since
the focus of the courses was not on software engineering
skills, the students could freely choose how to collaborate
on the assignments and we enforced no policy regarding to
collaboration or the commit messages.
B. Processing and Metrics
Before mining the raw data for metrics, we performed a
data cleaning step. We observed that students used different
machines with varying Git configurations for their work. This
resulted in multiple email identifiers for a student. Therefore,
we inspected the repositories and added .mailmap
files to
consolidate the different identifiers. Then, we did the data
mining with proprietary APIs of CodeScene
, a tool for
predictive analyses and visualizations to prioritize technical
debt in large-scale code bases. The tool processes the individual
Git repositories together with the information about the separate
assignments. We customized the analysis to calculate metrics
per assignment solution and selected the following metrics:
Number of commits.
The total number of commits
related to the specific assignment.
Mean author commits.
The mean number of commits
per author.
Mean commit message length.
The mean number of
characters in commit messages, excluding merge commits.
Number of merge commits. The number of merges.
Number of bug fixes.
The number of commits with
‘bugfix’ or ‘fix’ in the commit message.
Number of refactorings.
The number of commits with
‘refactor’ or ‘improve’ in the commit message.
Author fragmentation.
A metric describing how frag-
mented the work on single files is across authors [5].
Days with commits.
The number of days with at least
one commit in the assignment period.
These metrics cover the most relevant aspects of the process.
Unfortunately, we could not consider the size, i.e., the number
of additions and deletions of the commits, because the students
Fig. 1. Distribution of points per courses and assignments
imported project skeletons for each assignment and there were
dependencies between the assignments. For example, in PKS
the very same task had to be solved using different technologies
and the students were encouraged to copy their old solution to
the current assignment for comparing the performances.
Before investigating the code process metrics, we display the
distribution of achieved points per course and assignment in
Figure 1. In AJP, the median score of
is quite high and
homogeneous over the course, however, there is some variability
with a standard deviation of
. In the more advanced PKS
course, the average score had a rising tendency with a median
value of only
in the first assignment until a median of
in the fourth. We assume that this is due to students
having little prior knowledge in concurrency programming.
Additionally, the first assignment deals with low-level threading
mechanisms, which require a profound understanding. The
students gradually improve their performance over the course
by gaining experience and because the later assignments deal
with more convenient concurrency programming constructs.
The standard deviation of points is 1.88.
A. Correlating Code Process Metrics with Points
To analyze the aforementioned code process metrics for
correlations with the achieved points, we calculated the pairwise
Pearson Correlation Coefficient (PCC) between all features over
all solutions irrespective of the course. Surprisingly, we did not
encounter any notable relationship between any of our metrics
and points, as can be seen in the ‘Overall’ column of Table II.
Feature Overall AJP PKS
Mean Author Fragmentation 0.01 0.09 -0.25
Mean Commit Message Length 0.20 0.35 -0.06
Mean Author Commits 0.05 0.04 -0.09
Number of Commits 0.04 0.05 -0.16
Number of Merge Commits 0.08 0.10 -0.12
Number of Bug Fixes 0.04 0.09 -0.04
Days With Commits 0.03 0.07 -0.11
Number of Refactorings 0.07 0.07 0.08
24ISEE 2019: 2nd Workshop on Innovative Software Engineering Education @ SE19, Stuttgart, Germany
Fig. 2. The score relative to the number of commits
When looking at the two courses separately, however, in
AJP, one can solely see a moderate positive correlation of
between the commit message length and points. Interestingly,
this effect cannot be seen in PKS. There, we mainly see weak
negative correlations, which is also surprising, as it seems that
in contrast to AJP, more work does not lead to more points.
More effort does not necessarily mean more points.
While it is generally hard to directly quantify the effort using
our metrics, the combination of the number of commits and
the days with commits are the best available proxy. Figure 2
shows the number of commits per course and assignment. The
lines drawn on top of the data points are a linear regression
model that serves as a visual aid for the detailed trends in
the data. Interestingly, there is no consistent trend observable
over the assignments or the course. AJP Assignment 1 has a
negative trend, indicating that those groups that managed to
solve the assignment with fewer commits got higher scores in
that assignment. We attribute this to the prior knowledge of the
students at the start of the course. In the next two assignments
of AJP, the trend is positive, whereas the number of commits
in the last assignment did not have an impact on the grading.
In PKS, we see positive trends between both the number of
commits and days with commits with the points in the first
three assignments, while the last assignment shows a flat trend.
Our interpretation of this matter is the following: Assignments
that require much code to be written by the students benefit
from more commits, while it is the other way for assignments
where the framework guides the development. Recall that in
Assignment 4 of AJP, the task is to write a GUI using JavaFX,
and Assignment 4 of PKS is about using
Distributing the work over a longer time span does
not increase the points
. Generally, we assumed that starting
earlier and constantly working on the assignments, therefore,
accumulating more days with a commit would increase the
score. However, this is not the case. In AJP, the PCC between
this feature and points was
, whereas in PKS it was even
a weak negative value of
. We assume this has to do
with the limited temporal scope of only two weeks of working
on the assignments. The more experienced groups might have
finished the assignment in a shorter time period and stopped
working when they thought that their solution was sufficient.
Fig. 3. The score relative to the author fragmentation
Working on the same classes is not advisable.
metric we analyzed was the author fragmentation. It measures
if the Java classes were written by a sole author (zero
fragmentation) or collaboratively. In AJP there was again barely
a correlation of
, whereas in PKS there was a weak negative
PCC value of
. This is somewhat in line with findings
in literature, where a lower fragmentation indicates a higher
quality of the software [5]. When we have a closer look at the
assignments in Figure 3, however, there is again a mixed signal
of PKS Assignment 1 and 4 having a negative dependency,
whereas Assignment 2 and 3 are relatively stable.
Finally, we refrain from analyzing the number of bug fixes
and refactorings because they were rarely used in the commits.
B. Discussion
We found no notable correlations between the analyzed code
process metrics and the quality of the assignments measured
via manual grading. This stands in contrast to the literature on
software quality and code process metrics in industry [6]. So
what makes the assignments different from real world projects?
First of all, the timeframe differs. The assignments in our
courses all lasted for two weeks, whereas projects in industry
span multiple months and even years. Furthermore, students
focus solely on developing software, giving little thought on
how to run and maintain their software. What is more, the
students had a good feeling when their assignment met the
functional requirements and stopped working when they were
satisfied with their solution. Thus, equating the assignment
scores with the notion of high-quality software is most probably
not permissible in our courses.
On the other hand, it might be that code process metrics
simply require a certain effort by more developers to be put into
the code, which is not done by the small student groups during
the short assignment period. In industry projects, maintenance
work, i.e., bug fixing and refactoring, accounts for a large
portion of the commits and overall quality of the software. By
looking at the commit messages of the student assignments, we
see that such efforts were rare. Also, communication problems
become more of an issue in larger groups.
Finally, the student groups were quite new to the management
of group programming tasks, especially in the third semester
25ISEE 2019: 2nd Workshop on Innovative Software Engineering Education @ SE19, Stuttgart, Germany
course AJP. Since they could organize the development on their
own, there were myriads of different strategies. We believe
that this lack of organizational requirement is a key point in
why we don’t see clear patterns in the code process metrics.
Our approach is a contribution to learning analytics for
which Greller et al. name two basic goals: prediction and
reflection [7]. The commit data we analyzed has a coarse
granularity compared to other work on programming education
reviewed by Ihantola et al. [8], where the level of analysis is
typically finer, for example key strokes. Our initial hope was
that code process metrics could have some predictive power
for student courses. This, however, was not the case despite
several studies related to the quality and evolution of software
in industry [9]. Nagappan et al. found that the structure of
the development organization is a stronger predictor of defects
than code metrics from static analysis [6], and Mulder et al.
identified several cross-cutting concerns of doing software
repository mining [10]. This paper is, thus, a parallel approach
to static code analysis [2] or extensive test suites [11] for the
evaluation of student assignments.
The metrics used stem from the work of Greiler et al. [12],
D’Ambros et al. [1], and Tornhill [3], [13]. As an example, in
industry, the author fragmentation [5] is negatively correlated
with the code quality. This is supported by Greiler et al. [12],
who find that the number of defects increases with the number
of minor contributors in a module and Tufano et al. [14],
who find that the risk of a defect increases with the number of
developers who have worked on that part of the code. However,
one can also go further and look at the commit metadata to
capture the design degradation, as Oliva et al. did [15]. Our
approach therefore combines learning analytics with insights
from industry. Since in realistic projects a developer rarely
programs alone, we found that the focus of our analysis should
also be groups. This naturally limits us in drawing conclusions
about the learning process of an individual student.
While static code analysis has often been investigated in
educational settings, code process metrics from Git commits
with a focus on groups represent a novel direction. We present
an approach for analyzing code process metrics based on
Git commits from student assignments. However, from the
interpretation of our results, we cannot identify any metric
that has a significant correlation with the assignment scores
achieved by the students. Does this mean that code process
metrics are not useful for teaching programming? From our
experience, it is quite the contrary: We assume that the two
courses were not a realistic setting for profiting of good coding
practices. To be good software engineers in industry, students
should learn how to write maintainable code, even if their code
will be trashed after the semester. To establish good practices,
code process metrics should play a larger role in practical
software engineering courses, and could even be part of the
grading. In any case, in pure programming courses with very
limited timeframes code process metrics should not be used
for the assessment of assignment solutions, since they are bad
predictors for the score. Furthermore, when giving students
guidance about how to work on programming assignments, we
can give suggestions such as to start early, prefer more small
commits over fewer large commits, clearly separate tasks, but
they do not necessarily result in a better score.
We see time as a critical factor for the significance of
code process metrics. Future work could therefore analyze
development efforts with varying time frames to investigate
our argument. Our paper is a first attempt at utilizing code
process metrics in programming education impacted by the
characteristics of the courses we considered. This means there is
still potential in this topic and more research including different
contexts, especially larger student projects, is desirable.
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26ISEE 2019: 2nd Workshop on Innovative Software Engineering Education @ SE19, Stuttgart, Germany
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