Showing posts with label education. Show all posts
Showing posts with label education. Show all posts

PGM-class and MRF parameter learning

I’m taking Stanford CS 228 (a.k.a. pgm-class) on Coursera. The class is great, I guess it provides close to the maximum one can do under the constraints of remoteness and bulkness. The thing I miss is theoretical problems, which were taken aside from the on-line version because they could not be graded automatically.

There is an important thing about graphical models I fully realized only recently (partly due to the class). This thing should be articulated clearly in every introductory course, but is often just mentioned, probably because lecturers consider it obvious. The thing is there is no probabilistic meaning of MRF potentials whatsoever. The partition function is there not only for amenity: in contrast to Bayesian networks, there is no general way to assign potentials of an undirected graphical model to avoid normalization. The loops make it impossible. The implication is one should not assign potentials by estimating frequencies of assignments to factors (possibly conditioned on features) like I did earlier. This is quite a bad heuristic because it is susceptible to overcounting. Let me give an example.

For the third week programming assignment we needed to implement a Markov network for handwriting OCR. The unary and pairwise potentials are somewhat obvious, but there was also a task to add ternary factors. The accuracy of the pairwise model is 26%. Mihaly Barasz tried to add ternary factors with values proportional to trigram frequencies in English, which decreased performance to 21% (link for those who have access). After removing pairwise factors, the performance rose to 38%. Why has the joint model failed? The reason is overcounting evidence: different factor types enforce the same co-occurrences, thus creating bias towards more frequent assignments, and this shows it can be significant. Therefore, we should train models with cycles discriminatively. 

One more thought I’d like to share: graphical model design is similar to software engineering in the way that the crucial thing for the both is eliminating insignificant dependencies on the architecture design stage. 

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[CFP] GraphiCon-2011 and MSCVSS

Our lab traditionally organizes GraphiCon, the major (and may be the only) conference in Russia specialized in computer graphics and computer vision. The conference is not very selective, still it has a decent community of professionals behind it. So, if you want to get a quality feedback on your preliminary work, or always wanted to visit Russia, consider submitting a paper by May, 16.

Special offer for young readers of my blog: If it is the place where you learned about the conference and decided to submit a paper, I'll buy you a beer during the conference. :) Consider it another social micro-event.

Another piece of news is primarily for undergrads and PhD students from Russia. Our lab and Microsoft Research organize another event this summer: Computer vision summer school. There are such lecturers as Cristoph Lampert and Andrew Zisserman, among others. Participation is free of charge, and accommodation is provided. Deadline is on April, 30. You should not miss it!

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ICVSS 2010: some trivia

If you think that summer schools are only about lectures and posters, you are certainly wrong. Well, I also did not expect that I would get drunk every night (it was not in Russia after all!). On the first night I convinced people to go to the beach for night swimming, but on the next day I realized that it was not a really good idea, because lectures started at 9. Eventually, we did not go to bed early for the rest of the week.

I would like to thank "Marsa Sicla" people (Fig. 1), you were the great company!

Fig. 1. The Marsa Sicla company. Left to right: Richard, Ramin,
Nicolas, Xi, Vijay, Clément, Aish, me. Sandra is behind the camera.

Actually, there was a reason why we had such a close-knit company. On the Fig. 2 you can see the map of the region where the school took place. The school was hosted by Baia Samuele hotel village (inside the blue frame on the map). The cheapest accommodation at Baia Samuele costed €100 per night, so some pragmatic people scared a different accommodation up. It was in a neighbouring hotel village, Marsa Sicla.


Fig. 2. ICVSS location map. View ICVSS_2010_MSvsBS in a larger map

In a map it looked pretty close, but in practice it turned out that there were a lot of fences (blue line)! Officially, we had to walk along the red route (~2.5 km, partly along a highway) through the official entrance. Moreover, we had to leave the village for lunch (since there were buffet), otherwise we would have been charged €30 per meal. To control it they wanted us to leave our documents at the control post (bottom blue tick on Fig. 2). Thus, we were supposed to walk 2.5 km four times a day. But we found a better solution.

Although the fence was equipped with a barbwire, we managed to find two shortcuts (green ticks). One of them led through ever-closed gate (Fig. 3), which were suitable for hopping over. So, we used the green path usually. It made the way two times shorter. Moreover, we did not have to leave Baia Samuele for lunch, since we weren't officially there. Sometimes we even had free lunch, that finally proved that the no free lunch theorem is wrong (joke by Ramin).

Fig. 3. Hopping over the fence.

Well, the lunch was really great. But this was not really a question of food, the main reason not to leave Baia Samuele was of course socialization, which used to be active during lunch (and also Internet access at the conference centre :). If we knew about such situation before booking the apartments, we would probably followed the official accommodation recommendations. However, we had a great company at Marsa Sicla too. But because of "unofficial night programme" I had to (almost) sleep during lectures. Most of guys was on their last year of PhD, so they considered the school as a vacation. But I really wanted to learn a lot of stuff. It was really difficult to perceive any information when you had not slept enough time. Surprisingly, I somehow managed to pass the final exam, but I feel I need to look through the lecture slides again, reading up some papers they refer to, otherwise the scientific part of the school will be useless for me.

Thus, I finish posting about ICVSS. To conclude, I want to recommend everyone to visit such summer schools, because they are the best way to enter into the community.

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University education reform

Recently, me and my friend Andrew Korolev came up with the plan of reforming Russian higher education system.


There are some evident problems in the system one can notice even in our faculty. The most courses are obsolete (or, well, legacy), a lot of courses are not enough developed (e.g. they contain only lectures without any support), some of them are also over-theorized and not applicable in real world (or at least they don't touch upon their applications). As a result, students are not motivated, they don't attend lectures and learn everything during the two-day period prior to the exam. The grades are often lousy, but nobody cares since such grades are sufficient to continue studying. (It is difficult to fail totally, especially for non-freshmen). Such knowledge is not solid and quite useless.

We make use of the following facts. The professors are well-qualified here, the students are witty and communicate with each other and with the graduates. Also, we suppose that everyone wants get more money, which is not always hold though is pretty common.

First, we need to improve the quality of courses. The problem is how to measure the quality. We state that it is proportional to the number of students who choose the course. Since the students have enough information about the course (from the lecturers and other students) and they all want to make a magnificent career, the useful courses become popular. The professors should be paid according to the number of students who attend their courses. But students also want to save their time. So, they are likely to choose the least challenging courses, which are not useful. In order to penalize them, the course part of professor's salary should be eliminated by the fraction of positive grades students get. Thus, a professor is motivated to make a comprehensive course (to attract students) and to implement a severe grade policy (to eliminate the freebie).

However, it will never work if the students are not motivated to get the good grades. In Russia, we have 4-level grades (2 through 5; 3 is enough for the pass), and most of the students are happy with 3s. Today, there are two stimuli to get greater grades, but they are quite subtle. The first is one need almost all 5s to receive the degree with honours, but who needs that? The second is 20% increment to the scholarship, which amounts not as much as one can desire. I have all excellent grades and receive some personal Sberbank scholarship, and totally it is about €100 per month. So, this is not a stimulus.

Well, one can ask, why do the students study now, if the courses are far from perfect? The answer is the conscription. In Russia, while you keep studying, you have a delay. If you fail at an exam, you get expelled and go to the army. Obviously, nobody wants to go to the Russian Army. So, everybody can learn a bit to pass an exam. Our point is to motivate students this way: if you have lousy grades, instead of summer holidays you move to a military camp. Better grades you get, less term you should serve. Thus, students WILL get good grades! But it is hard to get them, because professors loose their money. It is kind of a dual problem.

Surely, the model is way rough and could not be applied directly. Moreover, it is too funny to be taken seriously, though, as Russian proverb says, every joke contains a bit of truth.

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