Wednesday, January 23, 2008

Lessons from Laundry

Several years ago, I was advising a student on some code he had written for a data structures class. This code performed a set of actions on a temporary counter, and he was resetting the counter to zero after the last action so it would be ready for the next cycle of actions. I suggested setting the counter to zero just before the first action instead of just after the last action. He asked why, and this was my explanation.

This is also an example of what Jeanette Wing and others call computational thinking.

You know when you're doing laundry and you have to clean the lint out of the lint trap? Of course, this is not mandatory—the dryer will still work if you don't clean out the lint, but it'll work more efficiently if you do.

Now, you have two basic choices as to when to clean out the lint. You can clean it out when you remove clothes from the dryer, or you can clean it out when you put clothes into the dryer. Either protocol will work fine as long as everybody in the house is following the same protocol.

But suppose you have two people in the house following opposite protocols. Let's say that you follow the clean-after protocol and your housemate follows the clean-before protocol. If you do a load of laundry and clean out the lint trap when you're done, and then your housemate does a load of laundry, then the worst that will happen is that the lint trap will already be clean when your housemate goes to clean it.

On the other hand, if your housemate does a load of laundry and cleans out the lint trap at the beginning, and then you do a load of laundry, now you have a problem. When you go to clean out the lint when you're done, you'll find it extra full. You might even need to run the dryer a second time because your clothes didn't get completely dry.

The point is that some protocols, such as the clean-after protocol, only work if everybody follows the same protocol. Other protocols, such as the clean-before protocol, are more robust; they work fine—at least for you—even if some people follow the other protocol. If you can't guarantee that others will follow (or even remember!) a particular protocol, then you're better off choosing the robust protocol.

Here's a similar example. In college, I used to play a lot of bridge in the dorm lounge. In the hall right outside the lounge, there was a small bathroom. Now, there were two protocols concerning this bathroom. In the first protocol, you lock the door when using the bathroom; in the second protocol, you knock before entering. Somebody following the locked-door protocol may or may not knock, while somebody following the knocking protocol may or may not lock the door.

As long as everybody in the dorm follows the same protocol, there's no problem. But if different people follow different protocols, there can be tragic (or at least embarrassing) consequences!

In this example, perhaps the best course of action is to follow both protocols: lock the door and knock. The same is possible in many other situations where you want to be extra careful: clean the lint after drying but also check it again just before drying, initialize the counter just before the first action but also reset it to zero just after the last action, wear both a belt and suspenders.

Friday, January 18, 2008

Why I Don't Use PowerPoint For Teaching

This essay was originally written for a talk I was giving to brand new faculty members. I'll let it stand as an introduction to one of my passions—teaching. I'll introduce other passions in the coming weeks. Welcome to my blog!

Ok, I’m a freak. I admit it. I don’t use PowerPoint for teaching. Well, hardly ever. Once or twice a semester. Around my institution—and across the country—that puts me in the tiny minority. I know this because my students find it unusual enough to comment on. (Update: Since I wrote this, PowerPoint use in the classroom appears to be on the decline, at least at my institution. Hooray!)

Why do I take this heretical position? Part of it is that PowerPoint doesn’t mesh well with my personal teaching style. But mostly it’s because PowerPoint is just too hard for me. Oh, not making slides. That part’s easy. I mean that creating a PowerPoint presentation that effectively supports my goals in the classroom is too hard. It’s way too much work. Some people can—I tip my hat to them—but me? I’m just not good enough to do that.

I’m sure you’ve seen through my little rhetorical device to the arrogance that lies beneath, the arrogance that says “I think I’m pretty good, so if I’m not good enough, then I think most other people aren’t either”. But what makes using PowerPoint for teaching so hard, when it seems so seductively easy?

Well, that’s exactly the problem. PowerPoint is seductively easy. The problem is that PowerPoint makes it easy to do the wrong things.

Here’s an analogy. If you have taught beginner programming classes, you have seen badly indented code. Why? Because in an editor that does not automatically indent, it is easier to write badly indented code than to indent properly. (At least in the short term. Most students don't believe us when we tell them that indenting their code properly will actually save them time in the long run.)

With PowerPoint, however, the situation is more subtle. That’s where the “seductive” part comes in. With indentation, the student usually knows that he is doing the wrong thing, but does it anyway. With PowerPoint, the instructor probably sincerely believes he is doing the right thing. The road to hell is paved with good intentions…

Let’s look at five ways that PowerPoint makes it easy to do the wrong thing.


The original name of PowerPoint was Presenter and that’s exactly what PowerPoint was designed for—presentations. Think about what that implies. One person (the presenter) is presenting information to other people (the audience). The flow of information is one way, from the presenter to the audience. Because the flow of information is one way, the presenter can and does script out the entire presentation ahead of time, much like a movie or a novel. Like those forms, a PowerPoint presentation is highly linear. It is meant to be experienced in a particular order. Deviating from the expected order is possible, but awkward. This model has little room for interactivity, except perhaps a single slide at the end labeled “Questions?”.

There are contexts where this linear model may be appropriate. You’ve probably seen presentations like this at business meetings or at research conferences. But the purpose of those presentations is not teaching. The goal may be to inform, to persuade, maybe even to train, but the goal is not to educate. What’s the difference? In a presentation, you are trying to make the audience think your thoughts, but in education, you are trying to teach students to think for themselves. In terms of the well-known fishing analogy, it’s the difference between giving somebody a fish and teaching them how to fish.

To teach students to think for themselves, you must give them plenty of opportunities to think for themselves and then respond to those thoughts (or allow others to respond). But this breaks the model of information flowing in one direction in a linear order. Instead you now have information flowing in both directions—a feedback loop—and the order of information can and will change based on that feedback. Good teaching is highly interactive. A good teacher is highly adaptive, and can change the entire direction of a lesson midstream based on student input. But, if you’re using PowerPoint, how do you change direction midstream? It’s hard to tell students “Just wait a few minutes while I edit these slides!”

You may remember the “Choose Your Own Adventure” books that were popular twenty or thirty years ago. You would read a page, and at the end of the page would be some choices. “If you pay the troll and cross the bridge, go to page 117. If you try to cross without paying, go to page 98. If you attack the troll, go to page 140.” These books were an attempt to shoehorn an interactive form into an inherently linear medium. It was an uncomfortable fit, and these books were soon replaced by computer programs that hid the linearity. It is certainly possible to design a PowerPoint presentation that supports interaction in this fashion, but, like the Choose Your Own Adventure books, it is always an uncomfortable fit.

So this is the first way that PowerPoint makes it easy to do the wrong thing. PowerPoint makes it easy to create a linear presentation, but hard to create an interactive lesson.

“It’s only wafer thin”

There is a scene in Monty Python’s The Meaning of Life in which, at the end of a ridiculously large meal, the maitre d’ offers a diner a mint. At first, the man refuses, but the maitre d’ talks him into it, saying “It’s only wafer thin” and later “Oh, sir, just—just one.” The man eats the mint and literally explodes. (There’s also a brilliant send up of this scene in the comic strip Foxtrot, where a girl wakes up with a swollen head after cramming for finals, and her brother taunts her, saying “It’s only a wafer-thin math formula!”)

This is the second way that PowerPoint makes it easy to do the wrong thing. There’s always the temptation to add just one more word, just one more bullet, just one more slide. It’s so easy to do, and, after all, wouldn’t the students be better off with more information, with more complete slides?

Well…no. Students have a limited capacity to absorb information from slides. Exceed this capacity, and they not only fail to absorb the excess, they also fail to absorb or retain what came before. This can happen either when a single slide contains too much information or when the presentation as a whole has too many slides.

Think of that limited capacity in terms of juggling. Most people can learn to juggle three balls with a little practice, but juggling four is much more difficult. So there you are, happily juggling three balls, when somebody tosses you another ball. What happens? Do you just drop the new ball and continue juggling the three you already have? No. What really happens is you drop all four balls.

Teachers often prepare extra slides “just in case there’s extra time” or “just in case somebody asks”. After all, as discussed above, you want to be prepared to respond to student questions. That’s fine, as long as you are strong-willed enough to resist the temptation to show the extra slides. But, all too often, once you’ve invested the time and effort to create a slide you’re proud of, the psychological pressure to show that slide is irresistable.

Extra slides also cause awkward navigation issues. Where do you put the extra slide? Do you put it in the middle of the presentation, right where it might be needed? If so and it turns out not to be needed, then you have to hurriedly skip over the slide when it comes up, which never looks good. Or do you put it out of the way at the end of the presentation? But then, if it is needed, you have to skip through possibly dozens of slides to reach the extra one, and then go backwards through the same slides to get back to where you were. There are technical ways around these problems, but hardly anybody uses them.

He who controls the clicker, rules the world

Have you ever fought with a sibling or spouse or significant other over who gets to hold the TV remote control? This can be particularly contentious when you intend to channel surf, but the issue arises even if all you intend to do is watch a DVD. Why bother to fight over the remote in that situation? Because the person who holds the remote has enormous power. Another viewer with even the smallest request must approach as a humble supplicant—“please hit pause”, “please turn the volume up”, “hold on, I missed that, can you please go back a little bit?”

Teaching from PowerPoint slides is like holding the TV remote control (and, in fact, you may be literally holding a projector remote control). The teacher decides what slides to show and what to say about each, when to advance and when to go back, when to turn off the screen and when to turn it on. The teacher is in complete control. This is the epitome of teacher-centered learning, and is the third way in which PowerPoint makes it easy to do the wrong thing.

Especially to new teachers, being in complete control may sound like a good thing. The teacher's the one who knows what he's doing—of course the teacher should be in control! So then why does our dean's vision explicitly state that “Teaching is student centered and encourages active learning”?

Sure, the phrase student centered has been overused to the point of becoming educational gobbledygook, but there is an important idea there. The ultimate goal in any classroom is not for the instructor to teach, but for the student to learn. So shouldn’t attention be on the student learning? In a PowerPoint presentation, the attention is mostly on the teacher—the instructor is (mostly) concentrating on his own performance and the students are (hopefully) paying attention to that performance. Here’s a simple rule of thumb for you: If you’re not paying more attention to the students than they are paying to you, then your class is not student centered.

I always allow students to bring a single page of notes to exams. I encourage them to prepare the notes themselves, because the biggest benefit of the notes lies not in having the notes on the exam but rather in the cognitive act of organizing the information. Paradoxically, students who prepare their own notes often find that they never once refer to them during the exam, because the act of preparation was enough to get the information into their heads. On the other hand, students who use other people’s notes often find them useless on an exam because they don’t really understand how the information is organized.

This highlights the flaw in teacher-centered instruction, especially instruction based around PowerPoint. In creating the slides, the teacher is imposing his own organization of the information on the students, when he should be helping the students to organize the information for themselves.

Active learning

My friend Susan doesn’t like to listen to books on tape in the car, because inevitably she will zone out for a while and have to back up the tape. In a PowerPoint slide show, students also will inevitably zone out for a slide or two (or six). But when (if!) their attention wanders back to you, they probably will not ask you to back up the presentation a few slides. Not only will they have lost any chance of learning the content of the slides they missed, but now they probably will not have the context to be able to learn anything from the upcoming slides. And so the lost students will happily return to daydreaming. This is one danger of passive learning and why lectures are so often condemned. In a classroom where students are allowed to be passive observers, they need only keep reasonably attentive expressions on their faces and the instructor will never realize that no learning is taking place (or at least not until the moment has long passed).

This is the fourth way that PowerPoint makes it easy to do the wrong thing. The more you rely on slides, the more passive the students become.

Contrast a lesson where students are passively observing a PowerPoint presentation with a lesson where students are actively engaged in solving a problem. How easy is it to zone out when you are sitting at a desk looking at a screen full of text as opposed to when you are, say, trying to design a solution on a whiteboard? Now the intructor has a better chance of noticing that a student has failed to learn some important point in time to do something about it. Even better, the student has a better chance of remembering the important points of the lesson because memory formation and retrieval depend at least in part on how many different parts of the brain are active. When a student is thinking, writing, drawing, explaining, and building, more parts of the brain are involved than when he is merely watching and listening.

Although I know of no studies that measure audience brain activity during a PowerPoint presentation, I wouldn’t be surprised if it resembled the relatively flat EEGs found when people watch TV.

“It’s a floor wax and a dessert topping”

Although I almost never use slides in the classroom, I do use them for giving research talks. I am often approached and asked “I missed your talk. Can I get a copy of your slides?” The other person is usually shocked—and initially annoyed—when I say no. But they usually understand when I explain “I wrote the slides to accompany my talk. They wouldn’t make any sense without my words to go along with them. You’re better off looking at my paper, which was intended to be read on its own.”

A trap that instructors often fall into is putting their PowerPoint slides on the web. They feel virtuous in doing so, patting themselves on the back for helping out students who missed class and for giving students something to review later. This is the fifth way that PowerPoint makes it easy to do the wrong thing.

But how can it possibly be the wrong thing to give students your slides? Because slides that were designed for use in the classroom probably will not work well when viewed from a student's room, and slides that were designed to be viewed outside of class almost certainly will not work well in the classroom. Like the combination floor wax/dessert topping from the classic Saturday Night Live skit, it is nearly impossible to serve both purposes well—you can’t serve two masters. Chances are high that if you try to serve both purposes well, you’ll fail at both.

So perhaps you decide that you’re going to focus on making slides that work well in the classroom, and accept that maybe they won’t work so well from the student's room. Wouldn’t it still be better to give students the slides? Wouldn’t ineffective slides be better than nothing? Not necessarily. Not if the main outcome is to give students a false sense of security and to prevent them from seeking more effective means of learning. A student faced with the choice of reviewing bad slides, talking to a buddy about their notes, unsealing the shrinkwrap on the textbook, or coming in to see the teacher or TA will all too often choose the slides because they seem the easiest.

Going the other direction and deciding to focus on making slides that stand alone when read from the student's room is even worse. That way lies the kinds of overfull slides that leave students muttering “Death by PowerPoint”. Furthermore, students actively resent sitting through a 50 minute lecture when they feel they could have read through the slides in 10 minutes and gotten just as much out of it—especially when that impression is accurate!

In fact, the situation is even worse than I’ve described because instructors often try to create slides that serve more than two purposes:

  • visual aids for during the lesson
  • make-up material for students that missed the lesson
  • review for students that were present at the lesson
  • handouts for students during the lesson
  • read-ahead before the lesson
  • notes to the instructors themselves

I don’t care if you are Socrates, Halliday&Resnick, and Edward bloody Tufte all rolled into one. Try to do all this with one set of slides and you will fail. Decide which purpose you are trying to serve and serve it well.

Searching for the harder right

Make us to choose the harder right instead of the easier wrong.
– USMA Cadet Prayer

I've shown five ways in which PowerPoint encourages you, subtly or not so subtly, to choose the easier wrong. But what then is the harder right? Sorry, I can't tell you. Good teaching is hard, and part of what makes it hard is that it is both highly personal and highly context dependent. My solution wouldn't necessarily be right for you.

I admit that's a cop out, but it's also true. For some of you, PowerPoint itself might be the harder right. If you use it carefully and in moderation, with an eye out for the kinds of traps described above, you can probably do fine. But it sure is a lot more work that way, isn't it?