The concise answer is none. Duke, as you are aware, is extremely selective. Therefore, there is no systematic approach to "weed down" the incoming undergraduate class. This does not suggest that various classes are not difficult, nor that some fields-of-study (pre-med is a good example) do not experience attrition. However, unlike many less selective universities, 95+ percent of each year's freshmen graduate.
TopTier, I think you misunderstood. joeahn isn't asking for courses that thin out the overall undergraduate class, he's asking for courses essential to certain majors/tracks whose curve/difficulty causes many students to drop those majors/tracks, thus thinning out the ranks of students in a particular department or program.
A good example would be EGR53 or Math 103 causing many people to realize they don't like engineering and transfer to Trinity, thus "weeding out" the engineering class. Another would be orgo, a required premed class, which causes many to drop premed.
Those weeded out students still graduate from Duke but just not with the major or track that they started out in or came in wanting to do.
TopTier: I wouldn't say that, not trying to argue with you, but I've never heard of anyone at Duke or elsewhere use "weed out class" to refer to anything that suggests the purpose as forcing students to drop out of school altogether and as a premed, I can assure you, we talked about this stuff ad nauseam. Maybe at ETH or some other school in Europe but that's a whole different education model altogether.
Some people say econ is a weed out class, but it's not supposed to weed you out of Duke! Tons of people come here saying they want to do econ so it's more meant to get rid of people who aren't fully committed to the rigor of the program and choose other majors.
SBR: At many large, public universities "weed out classes" are/were quite common and specifically designed to drop a large percentage of freshman early in their undergarduate program. For example, a few decades ago, universities like Ohio State had an introductory and mandatory English Composition course that was failed by approximately a third of the students, who were immediately disenrolled.
TopTier, that's fine, but this is the Duke subforum where the question was asked and we must read in context. At Duke, weedout classes do not mean what they meant a few decades ago at large state universities like Ohio State. Just like the boot means the trunk in England. If I ask to put my stuff in the boot in London I don't expect to be shown a shoe and vice versa I would never ask that in New York.
I agree EGR103 is not intended as a weedout class and I do not mean to imply that it is made unnecessarily difficult to make it so.
However, it is the first class that engineers take and because of the subject matter, a highly technical introduction to computer science and engineering. For many students, this may be their first exposure to "engineering" classes beyond the standard math and basic sciences. And sad to say, some find that their first impression of engineering isn't what they imagined "engineering" to be and thus decide to transfer to Trinity. In this way, EGR103 does have the unintended effect of serving as a "weedout" class for Pratt.
SBR: are these courses considered "weed out" solely on the criteria of having difficult material/higher fail rates or are there other factors at play? Would going to office hours or getting help comprehending the material be useful or are these classes designed to not even offer help to those who seek it?
All professors teaching classes are expected to offer office hours or assign their TAs to conduct them on their behalf. Help is always available at Duke if students need it.
But as I alluded to before, while a "weed out" class is often challenging, it isn't always the case. Some "weed out" courses merely present new material which leads some students to decide that a course of study isn't for them. After all, it is always good to get an idea early on of whether or not you actually like economics/engineering/math/the sciences, etc.