#1




Calculating w from just support vectorsnumerically risky?
In a numerically perfect world, would be exactly 0 except at the support vectors, and so would give the same result is .
On real computers, of course, we have to deal with the fact that our calculations have limited precision, and so is usually nonzero nearly everywhere. I found that if I identified the support vectors before calculating , by looking for for some small , and then calculated just from those support vectors, I did not get a consistent . If there were 3 support vectors, sometime I'd get the same from all 3, but maybe half the time I'd get one from two of them, and the third would give a that was significantly off. If, however, I used all the vectors to calculate , rather than just the support vectors, then I'd get the same from all the support vectors. My speculation is that just as the values that are supposed to be 0 are off slightly due to floating point precision issues, so too are those that are supposed to be nonzero, and that when you use ALL of the 's to calculate the errors are balancing out. When you exclude the ones that were "supposed" to be 0, you increase the error in . This makes intuitive sense because the QP solver was using all the 's to try to achieve minimization, and so any error should be spread among all of them. If we only have 3 support vectors, and so only use 3 's, the error will be high because 3 is so small we get high variance. By using all the 's, the variance will be lower, and so the error is closer to the mean error, which should be zero. Those who had errors on problems 810, if you just used the support vectors, and calculated from one support vector, it might be worth putting in a check to see if you get a different from different support vectors. 
#2




Re: Calculating w from just support vectorsnumerically risky?
I was wondering why my results were sometimes weird... Indeed it seems that you have to use all the vectors even those who are "negligible" to obtain the best results.
I checked that the b was consistant among all vectors but only did it visuall once... I did not think of adding an automated test. (I still had the right answers for the homework though !) 
#3




Re: Calculating w from just support vectorsnumerically risky?
Quote:
In more specific SVM solvers (such as LIBSVM), support vectors are clearly identified (so there won't be the issue of ). Then, roughly speaking, is calculated from the average of possible that comes from every nonzero . The steps eliminates the effects of the numerical difficulty and that is why specific solvers are ofen preferred over generic QP solvers. Hope this helps.
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