My Grading Process

My GitHub activity, including this blog, has really slowed down for the past month because I’ve spent a lot of free time grading homework for a design patterns class, taught by a colleague at the Whiting School of Engineering. Conveniently for me, all of my interaction with the students is through e-mail. It’s been a great exercise of my new e-mail setup, which itself has definitely made this job easier. It’s kept me very organized through the whole process.

Each assignment involves applying two or three design patterns to a crude (in my opinion) XML parsing library. Students are given a tarball containing the source code for the library, in both Java and C++. They pick a language, modify the code to use the specified patterns, zip/archive up the result, and e-mail me their zipfile/tarball.

It took me the first couple of weeks to work out an efficient grading workflow, and, at this point, I can accurately work my way through most new homework submissions rapidly. On my end I already know the original code base. All I really care about is the student’s changes. In software development this sort of thing is expressed a diff, preferably in the unified diff format. This is called a patch. It describes precisely what was added and removed, and provides a bit of context around each change. The context greatly increases the readability of the patch and, as a bonus, allows it to be applied to a slightly different source. Here’s a part of a patch recently submitted to Elfeed:

diff --git a/tests/elfeed-tests.el b/tests/elfeed-tests.el
index 31d5ad2..fbb78dd 100644
--- a/tests/elfeed-tests.el
+++ b/tests/elfeed-tests.el
@@ -144,15 +144,15 @@
     (insert elfeed-test-rss)
     (goto-char (point-min))
-    (should (eq (elfeed-feed-type (xml-parse-region)) :rss)))
+    (should (eq (elfeed-feed-type (elfeed-xml-parse-region)) :rss)))
     (insert elfeed-test-atom)
     (goto-char (point-min))
-    (should (eq (elfeed-feed-type (xml-parse-region)) :atom)))
+    (should (eq (elfeed-feed-type (elfeed-xml-parse-region)) :atom)))
     (insert elfeed-test-rss1.0)
     (goto-char (point-min))
-    (should (eq (elfeed-feed-type (xml-parse-region)) :rss1.0))))
+    (should (eq (elfeed-feed-type (elfeed-xml-parse-region)) :rss1.0))))

 (ert-deftest elfeed-entries-from-x ()

I’d really prefer to receive patches like this as homework submissions but this is probably too sophisticated for most students. Instead, the first thing I do is create a patch for them from their submission. Most students work off of their previous submission, so I just run diff between their last submission and the current one. While I’ve got a lot of the rest of the process automated with scripts, I unfortunately cannot script patch generation. Each student’s submission follows a unique format for that particular student and some students are not even consistent between their own assignments. About half the students also include generated files alongside the source so I need to clean this up too. Generating the patch is by far the messiest part of the whole process.

I grade almost entirely from the patch. 100% correct submissions are usually only a few hundred lines of patch and I can spot all of the required parts within a few minutes. Very easy. It’s the incorrect submissions that consume most of my time. I have to figure out what they’re doing, determine what they meant to do, and distill that down into discrete discussion items along with point losses. In either case I’ll also add some of my own opinions on their choice of style, though this has no effect on the final grade.

For each student’s submission, I commit to a private Git repository the raw, submitted archive file, the generated patch, and a grade report written in Markdown. After the due date and once all the submitted assignments are graded, I reply to each student with their grade report. On a few occasions there’s been a back and forth clarification dialog that has resulted in the student getting a higher score. (That’s a hint to any students who happen to read this!)

Even ignoring the time it takes to generate a patch, there are still disadvantages to not having students submit patches. One is the size: about 60% of my current e-mail storage, which goes all the way back to 2006, is from this class alone from the past one month. It’s been a lot of bulky attachments. I’ll delete all of the attachments once the semester is over.

Another is that the students are unaware of the amount of changes they make. Some of these patches contain a significant number of trivial changes — breaking long lines in the original source, changing whitespace within lines, etc. If students focused on crafting a tidy patch they might try to avoid including these types of changes in their submissions. I like to imagine this process being similar to submitting a patch to an open source project. Patches should describe a concise set of changes, and messy patches are rejected outright. The Git staging area is all about crafting clean patches like this.

If there was something else I could change it would be to severely clean up the original code base. When compiler warnings are turned on, compiling it emits a giant list of warnings. The students are already starting at an unnecessary disadvantage, missing out on a very valuable feature: because of all the existing noise they can’t effectively use compiler warnings themselves. Any new warnings would be lost in the noise. This has also lead to many of those trivial/unrelated changes: some students are spending time fixing the warnings.

I want to go a lot further than warnings, though. I’d make sure the original code base had absolutely no issues listed by PMD, FindBugs, or Checkstyle (for the Java version, that is). Then I could use all of these static analysis tools on student’s submissions to quickly spot issues. It’s as simple as using my starter build configuration. In fact, I’ve used these tools a number of times in the past to perform detailed code reviews for free (1, 2, 3). Providing an extensive code analysis for each student for each assignment would become a realistic goal.

I’ve expressed all these ideas to the class’s instructor, my colleague, so maybe some things will change in future semesters. If I’m offered the opportunity again — assuming I didn’t screw this semester up already — I’m still unsure if I would want to grade a class again. It’s a lot of work for, optimistically, what amounts to the same pay rate I received as an engineering intern in college. This first experience at grading has been very educational, making me appreciate those who graded my own sloppy assignments in college, and that’s provided value beyond the monetary compensation. Next time around wouldn’t be as educational, so my time could probably be better spent on other activities, even if it’s writing open source software for free.

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null program

Chris Wellons (PGP)
~skeeto/ (view)