Gas station without pumps

2018 April 4

Quiz disappointment

Filed under: Circuits course — gasstationwithoutpumps @ 22:31
Tags: , ,

Today’s quiz in class was very disappointing for me, and I don’t know what to do differently to get better results.

Yesterday in lab I returned the quizzes from before break and urged students to look through all the old quizzes and rework any questions they got wrong, reminding them that I recycle questions.

In this morning’s class, before the quiz students asked me to show them (again) how to do one of the questions on the last quiz:

Design a voltage divider implementing Vout − Vref = G(Vin − Vref ), where G can be adjusted from ≈ 0.33 to ≈ 0.67 using a 10 kΩ potentiometer. Use port symbols to connect to Vin, Vref , and Vout.

I showed them two solutions—one that I had expected and a correct, but different solution that a student had come up with.

Here are the two solutions: mine on the left and an alternative one on the right.  (There is a third solution, similar to the second one, but with the potentiometer as variable resistor on the lower leg, rather than the upper one, and the two fixed resistors swapped.)

I had already shown them my solution a week and a half ago, right after they took the quiz, and I had posted both solutions on Piazza.  I not only showed the solution, but gave them an explanation of how it worked again and answered some questions students had about it (like why the gain was expressed the way it was with Vref, and why it was even considered a gain).

Right after that I erased the board and handed out the quizzes.  One of the quiz questions was the identical question that I had just worked on the board for them.  I was resigned to this being a free point for them (just like putting their section number on the quiz is a free point, which I use to distinguish those who are absent from those who are present but get no questions right).

But 20 of the students got no points for the question and 15 got only half credit (out of a class of about 72—there were supposed to be 79, but there were 7 students absent).  So almost half the class could not retain for 5 minutes a simple circuit that they should have been able to derive in a couple of minutes and which they had seen at least 3 times already.

Help!  During the last 10 weeks I’ve gone through just about all the ways I can think of to have the students understand voltage dividers and potentiometers, and I’m obviously not getting through to 28% of them (probably more, since the absentees are likely to be in the group that can’t do the problem also).

Does anyone have any useful advice?  (Giving up on the students is not useful advice—I want them to succeed.)


  1. This seems worth chasing down to find out more about what is going on. Possibly related is a lot of evidence that many people don’t actually read paragraphs of text, but instead use strategies learned for SATs etc where individual words are picked out and guesses made. If something similar is going on here — they aren’t actually dealing with the circuit at all, but just looking for clues to guess with — you might be able to tell by giving some of the class more decorated versions of the circuit (arrows to remind them of current flow, etc.).

    To make you feel better (or worse), my father left being a college prof (Physiology, Kinesiology) ca 1952 when — as he told me — he couldn’t figure out a grading scheme that would pass more than half his class! (He went into research …)

    Comment by alanone1 — 2018 April 4 @ 23:12 | Reply

    • I’ve heard from a couple of students that they knew what the circuit was they wanted, but were uncertain about the how to draw it. The mistake they made was to draw the output port in two places, corresponding to the two extreme positions of the wiper arm of the potentiometer, rather than connecting it to the wiper arm itself. I can see how this confusion can arise, but I would have expected it to be extinguished by now, after many weeks of seeing potentiometer circuits.

      I agree that many students do not seem to have attached any semantics to the schematics and are trying to memorize them as random collections of lines, triangles, and squiggles. This is akin to students memorize chunks of program code without understanding the semantics—the amount that can be memorized that way is small, and generalizing to bigger structures is very hard. I have been giving them more decorated versions of some of the circuits (I almost always add the current arrows for voltage dividers when I put them on the board, for example), but that doesn’t seem to have helped much.

      I’m trying to get students to decompose schematics into block diagrams, in order to be able to reverse the process and build schematics from block diagrams. Unfortunately, my book does not yet include exercises for analyzing circuits into block diagrams—something I’ll need to add this summer.

      Comment by gasstationwithoutpumps — 2018 April 8 @ 19:40 | Reply

  2. The quiz was too long. Took up valuable class time to do it, and show solutions. Although quiz material is important, it is important to find a balance. Currently the balance is off. Today I spend too much time trying to prepare gnuplot scripts for fitting different models, ultimately not being able to figure out how to do it after hours of searching the internet. I wanted to ask this question in class, but with 5 minutes left in class could not. This leads to points being taken off, and possibly lower quiz scores since time spend trying to prepare for the lab, could be spend understanding quiz material. Time is something that is valuable to students, especially engineering students, who can only designate a curtain amount of time to each class, since engineering classes are time consuming. Spending hours on technicalities, such as gnuplot, is not time well spend.

    Comment by firebender13 — 2018 April 5 @ 00:18 | Reply

    • I agree that Quiz 1 this quarter was too long. I’ll try to make the quizzes shorter in future, but I can’t guranteee I’l always succeed. Sometimes it is hard for me to estimate how difficult a question will be for the students when I’m writing it, as I have the answer already in mind then. That is one reason I work the exercises again to make the key while students are doing the quiz—it gives me a chance to time how long it takes to come up with a solution when it isn’t fresh in my mind.

      The main points of Lab 7 are to learn to use the oscilloscope and to fit models, so spending time on figuring that model fitting was not wasted, even if it was frustrating and slower than optimal. I wish I had been able to get to the model fitting on Wednesday rather than Friday, though. I did go over model fitting in my office hours Wed and Thurs and during lab time Thurs evening for students who asked, but those more individual presentations did not reach more than about 1/3 of the class.

      The extra time on quizzes this year came as a result of student feedback from previous years, when quizzes were much less frequent and longer (more like midterm exams). Students did very poorly on those quizzes also and suggested that having the quizzes be smaller and more frequent would give them more chance to learn from their mistakes. This sounded likely to me, so I’m trying the smaller quizzes with immediate feedback this year, plus recycling questions between quizzes, so students can learn from their mistakes, but a big fraction of the class is not learning even with this structure, and I’m not sure what else to try.

      Comment by gasstationwithoutpumps — 2018 April 8 @ 19:51 | Reply

  3. Looks similar to except that Mark Guzdial didn’t quiz the students again right afterward the explaining…

    Comment by Lex Nederbragt (@lexnederbragt) — 2018 April 6 @ 10:17 | Reply

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: