Is there a way to write a specific rule that will drop the lowest grade from an assignment group, but only if the assignment was attempted? I would prefer not to drop any assignment with a 0 as it seems to motivate students to skip the last assignment at the end of the semester knowing that the grade will be dropped.
Canvas will only drop grades that have been attempted. If there's no grade for it, then Canvas won't drop it. Leaving it blank will keep it from getting dropped, but it also will not count as a zero, either.
I don't follow what not putting a grade in has to do with students thinking they can still turn it in. If you don't want students to turn it in, then you use the "available until" date so that they can not turn it in. If you don't do that, then students can still turn something in, even if you don't want them to.
I see a potential problem with not dropping zeros, but your situation may be different than mine so it may not apply to you. In my case, students can, and often do, legitimately earn a 0. This is especially the case when they don't take a quiz and I put a 0 in there for them. But I intend to allow them one uh-oh per term, so I drop the lowest grade, even if it is a 0.
Now, back to your question. No, there is no rule that would allow you to only drop non-zero grades.
I am going to provide some additional resources a little later on, but I'd like to better understand the problem. It's not strictly necessary to answer the question, but the reason you've given may not justify the extra work involved.
If you promise students that you will drop the lowest grade, then what does it matter if they skip the last one because they've done well on all of the others? It seems like that is within their right and should be expected. I've heard of faculty making a contract with the students that if they do well on all of the assignments, then they can skip the final. I'm not advocating that, but it seems that students have the mentality that when they've worked hard all semester, and the teacher let other (implied poorer) students get a break, why shouldn't they get the advantage of that as well?
If the student does well all semester and turns in a really crappy final assignment and you give them even 1 point, then it's going to be dropped under your rules. What's the real difference between that and getting 0 points? They both get dropped and the student who has worked hard is afforded the same chance that everyone else is.
My lowest grade in grad school was in a class where the teacher let the students who did poorly on an assignment redo it. I didn't think I had the option since I hadn't done bad. It wouldn't have taken much to redo it and I never questioned the instructor about it, but I feel the negative after my grade was a reflection of that assignment. Because our experiences drive our beliefs and beliefs lead to actions, I cannot in my sense of fairness and equity, offer something to bad students or those who have a crisis, but not offer the same thing to the good students. Other people have different experiences and beliefs, but I can only speak from mine and try to understand other's.
In other words, this isn't a battle I would choose to fight. I would let the students have their brief rest at the end of the semester when everything seems to come due at once and they have other classes that might be more demanding than mine at the moment.
Most people probably believe there are exceptions to every rule. I teach math and we generally don't believe in exceptions, tending to believe that any exception was the result of poorly defined conditions and assumptions.
However, I will grant that there are legitimate cases where a faculty may not want to drop a 0, even though they're dropping the lowest grade in a category. I don't think discouraging students who have done well from slacking off is one of them, but again your use case is probably different than mine. In the past, some people wanted to not drop a 0 that was the result of cheating. Other people might be like you and promise your students that you will drop the lowest grade that they turn in, but you need to be sure that it's worded that way, otherwise they can fight you on it.
Regardless of the reason why you want to do this, here's a quick solution that you can use. It assumes that you're dropping just one grade from the assignment category.
Since the fake assignment has a high point value, a 0 there will cause Canvas to drop it since it would be most detrimental to the student's grade. Since they only get to drop one assignment, this means that Canvas will keep any other 0's that they have on the assignments. This is why it's important to go ahead and put the 0's in for the regular assignments.
By leaving the assignment blank for the other students, Canvas ignores it in any grade calculations.This means that it won't be dropped and the lowest regular grade will be dropped. It also means that it won't be included in the points, so making it a ridiculously high value won't matter except to scare the students.
The timing is up to you, but it is important that you do not adds zeros for that fake assignment until after you have added the rule to drop the lowest grade. If you add the rule at the start of the semester, then add the 0's from step 4 as soon as they get their first 0 on another assignment. If you add the rule to drop at the end of the semester, then add the 0's in that extra assignment at the end of the semester after you've added the rule.
Here's the extra information I promised.
There was a feature request for this in August 2018, but it got archived with 5 votes: Dropping Lowest Score different than Zero
You won't be able to read the comments there without joining the cold storage group, so I'll copy/paste some of the thread from there:
This idea has come up a few times before. I'm linking to some of those items, which ties them to this idea in case people there are interested in commenting or voting here. The justifications mostly talk about cheating, but the solutions would apply for your situation as well.
- grade of zero because of cheating (this has a simple work around, although you can just leave it blank for the other students rather than just assigning it to the one)
- Help with 'ignore lowest' rule in Gradebook? (this has a lot of good work arounds)
- Exceptions to dropping the lowest score (this idea is archived, but had some interesting discussion)
- It seems that somewhere that I can't find it right now, there was a request to add a new category to the gradebook that would be counted as a 0 and be ineligible from dropping, like ND or Z
I'd also like to point out another message Excusing a student from assignment *and* dropping the lowest score? There, it was discovered that Canvas was actually treating EX like a 0 and dropping it. I don't know if that is intended behavior or was a bug. That was almost a year ago and there haven't been any comments on it since. But it might impact some of the proposed solutions, so I wanted to throw it out there.
My personal take is that the methods described in the other discussions are probably more flexible than a simple "ignore a zero". A student could get a legitimate 0 with a submission and then it should be dropped.
Follow up: I know that I can leave the assignment blank instead of assigning a 0, but then students seem to think that it I will accept it late..................................