Blogging has been kind of slow due to the birth of my son 2 weeks ago. I’ll be back at it soon, I promise. In the meanwhile, let me briefly pause to wish a happy anniversary to On the Origin of Species, which was published 150 years ago today. I recommend that you celebrate by buying yourself a t-shirt.
Month: November 2009
A student’s perspective on PowerPoint lectures
A student blogger who goes by Carolyn Blogs has an interesting entry on PowerPoint lectures from the perspective of someone taking the class:
Recently I came to the conclusion that I do not learn well from classes in which the lectures are based on PowerPoint presentations… Professors who use PowerPoint tend to present topics very quickly when they don’t have to do anything but talk. If every example and every diagram is on the screen, there isn’t much time for me to take notes on the subject of each slide. Lectures aided by chalkboard visuals are easier to take notes from because I can write what the professor writes on the board at the same time. Also, because there is usually more chalkboard space than screen space, if I am behind on note-taking, the visual will probably still be on the board for me to copy a few minutes later. A lot of professors try to solve this problem by handing out the lecture slides before class, or by posting them online. While this is great for a lot of students, it doesn’t work for me because I learn best and am most engaged if I have to take notes as if my grade depended on having a great record of the class and I would never see the material again. In classes with handouts, I tend to zone out and have to work harder to pay attention. Studies have shown[pdf] that taking high-quality notes improves organic memory: I rarely use my notes after the lecture because the act of physically writing information down helps me remember more of what goes on in class.
A few years ago I started phasing out PowerPoint from my upper-division classes (I never used it for grad classes). Carolyn hits on pretty much all the major reasons.
Teaching with PowerPoint has a different pace and structure than teaching with chalk or markers. It’s not just about overall fast vs. slow (though that’s part of it), but about when you go fast and when you go slow. When I use the board, I write down the major points, terms, definitions, etc. That forces me to slow down at exactly the moment when I’m making a big point and students should be attending closely. Once the critical information is on the board, I can elaborate, discuss with the class, ask questions, etc. while it hangs up there behind me for students to refer to. And since writing slows me down, I don’t give as much emphasis to relatively minor points — giving students an additional cue as to what’s more and less important. (“Don’t ignore this completely, but it’s not as central as what I said earlier.”) You can reproduce this kind of pacing and structure with PowerPoint, but in practice it’s difficult to do during a live performance in front of a classroom. You have to write your presentation with delivery (not just content) in mind. Otherwise it’s just too easy to blow through major and minor points at a constant pace.
Another point that she makes… I still use PowerPoint in my big introductory classes (though I make my own slides from scratch, use animation to help regulate my delivery, and try to avoid the mind-numbing bullety templates). I always have a few students ask me to post the notes before class. I don’t — I post them after class, but honestly, I have sometimes wondered if I’d be better off not posting them at all. Carolyn modestly writes “while [posting notes] is great for a lot of students, it doesn’t work for me…” but I actually think this describes most students. A lot of students misread their internal cues — if it feels like they are expending a lot of effort then they think they must be struggling with the material. Actually, though, if the professor is presenting challenging material, then you shouldn’t feel relaxed — relaxation is a sign that you’re probably thinking superficially or zoning out, not that you’ve quickly mastered the material.
I also found it impressive that Carolyn reached this conclusion on her own. Because frankly, it’s fundamentally very difficult to introspect into your own learning processes. A few years back, when I started moving away from PowerPoint, I got feedback on my student evaluations from people who wanted more PowerPoint. When I talked with students who felt that way, they thought they’d be able to focus more on the material if they didn’t have to bother taking notes. I realized that reflects a fundamental misunderstanding of what note-taking does for you. I’ve been getting less of that feedback lately — maybe because I’ve gotten better at using the board, or maybe because recent students have been around PowerPoint longer and see its limitations more clearly.
Say it again
When students learn writing, they often are taught that if you have to say the same kind of thing more than once, word things in a slightly different way each time. The idea is to add interest through variety.
But when I work with psychology students on their writing, I often have to work hard to break them of that habit. In scientific writing, precision and clarity are the most important. This doesn’t mean that scientific writing cannot also be elegant and interesting (the vary-the-wording strategy is often just a cheap trick anyhow). But your first priority is to make sure that your reader knows exactly what you mean.
Problems arise when journalists trained in vary-the-wording write about statistics. Small thing, but take this sentence from a Slate piece (in the oft-enlightening Explainer column) about the Fort Hood shooting:
Studies have shown that the suicide rate among male doctors is 40 percent higher than among men overall and that female doctors take their own lives at 130 percent the rate of women in general.
The same comparison is being made for men and for women: how does the suicide rate among doctors compare to the general population? But the numbers are not presented in parallel. For men, the number presented is 40, as in “40 percent higher than” men in general. For women, the number is 130, as in “130 percent the rate of” women in general.
The prepositions are the tipoff that the writer is doing different things, and a careful reader can probably figure that out. But the attempt to add variety just bogs things down. A reader will have to slow down and possibly re-read once or twice to figure out that 40% and 130% are both telling us that doctors commit suicide more often than others.
Separately: why break it out by gender? In context, the writer is trying to make a point about doctors versus everybody else. Not male doctors versus female doctors. We often reflexively categorize things by gender (I’m using “we” in a society-wide sense) when it’s unnecessary and uninformative.
Causality, genes, and the law
Ewen Callaway in New Scientist reports:
In 2007, Abdelmalek Bayout admitted to stabbing and killing a man and received a sentenced of 9 years and 2 months. Last week, Nature reported that Pier Valerio Reinotti, an appeal court judge in Trieste, Italy, cut Bayout’s sentence by a year after finding out he has gene variants linked to aggression. Leaving aside the question of whether this link is well enough understood to justify Reinotti’s decision, should genes ever be considered a legitimate defence?
Short answer: probably not.
Long answer: This reminds me of an issue I have with the Rubin Causal Model. In Holland’s 1986 paper on the RCM, he has a section titled “What can be a cause?” He introduces the notion of potential exposability – basically the idea that something can only be a cause if you could, in principle, manipulate it. He contrasts causes with attributes – features of individuals that are part of the definition of the individual. He uses as an example the statement, “She did well on the exam because she is a woman.” Gender can be statistically associated (correlated) with an outcome, but it cannot be a cause (according to Holland and I believe Rubin as well), because the person who did well on the exam would not be the same person if “she” weren’t a woman.
From a scientific/philosophical level, I’ve never liked the way they make the cause/attribute distinction. The RCM is so elegant and logical and principled, and then they tack on this very pragmatic and mushy issue of what can and cannot be manipulated. If technology changes to where something becomes manipulable, or if someone else thinks of a manipulation that escapes the researcher’s imagination (sex reassignment surgery?), things can shift back and forth from being classed as causes versus as attributes. Philosophically speaking: Blech. Plus, it leads to places I don’t really like. What about: “Jane didn’t get the job because she is a woman.” Is Holland saying that we cannot say that an applicant’s gender affected the employer’s hiring decision?
I think we just need to be better about defining the units and the nature of the counterfactuals. If we are trying to draw inferences about Jane, as she existed on a specific date and time and location, and therefore as a principled matter of defining the question (not as a pragmatic concern) we take as an a priori fact that Jane for the purposes of this problem has to be a woman, then okay, we’ve defined our problem space in a particular way that excludes “is a man” as a potential state of Jane. But if we are trying to draw inferences in which the units are exam-takers or job applicants, and Jane is one of many potential members of that population of units, then we’re dealing with a totally different question. In that case, we could have had either a man or a woman take the exam or apply for the job. Put another way: what is the counterfactual to Jane taking the exam or Jane applying for the job? If Jane could have been John for purposes of the problem that we are trying to solve, then it makes perfectly good sense to say that “Jane did well on the exam because she is a woman” is a coherent causal inference. It goes back to a principled matter of how we have defined the problem. Not a practical question of manipulability.
So back to the criminal… Holland (and Rubin) would make the question, “Is the MAOA-L variant a cause or an attribute?” And then they’d get into questions of whether you could manipulate that gene. And right now we cannot, so it’s an attribute; but maybe someday we’ll be able to, and then it’ll be a cause.
But I’d instead approach it by asking: what are the units, and what’s the counterfactual? To a scientist, it makes perfect sense to formulate a causal-inference problem in which the universe of units consists of all possible persons. Then we compare two persons whose genomes are entirely identical except for their MAOA variant, and we ask what the potential outcomes would be if one vs. the other was put in some situation that allows you to measure aggressive behavior. So the scientist gets to ask questions about MAOA causing aggression, because the scientist is drawing inferences about how persons behave, and MAOA is a variable across those units (generic persons).
But a court is supposed to ask different kinds of causal questions. The court judges the actual individual before it. And the units are potential or actual actions of that specific person as he existed on the day of the alleged crime. The units are not members of the generic category of persons. Thus, the court should not be considering what would happen if the real Abdelmalek Bayout had been replaced by a hypothetical almost-Bayout with a minutely different genome. A scientist can go there, but a court cannot. Rather, the court’s counterfactual is a different behavior from the very same real-world Abdelmalek Bayout, i.e., a Bayout who didn’t stab anybody on that day in 2007. And if Bayout had not stabbed anybody, there’d be no murder. But since he did, he caused a murder.
Addendum: it’s a totally different question of whether we want to hold all persons to the same standards. For example, we have the insanity defense. But there, it’s not a question of causality. In fact, defendants who plead insanity have to stipulate to the causal question (e.g. in a murder trial, they have to acknowledge that the defendant’s actions caused the death of another). The question before the court basically becomes a descriptive question — is this person sane or insane? — not a causal one.