Zeroed weights for entire class about libsvm HOT 6 CLOSED

The per-class weight is multiplied with the C value to obtain the penalty factor for a class. Setting the weight for label 2 to zero makes that class unweighted in the optimization problem. This in turn means that during the training procedure, classifying wrongly as class 2 is not penalized. Minimizing the objective during training can therefore be accomplished by assigning all samples to that class.

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libsvm uses 1-vs-1 for multiclass. For each binary problem, if one class has
C=0, then the whole alpha vector is zero for both classes due to the
y^T alpha = 0 constraint. So libsvm fits
neither class in this situation. Therefore, we cannot just think that in your
case only class 2 is not fitted. After pairwise training, there is another
stage of combination. All together the behavior can be very unpredictable
so your situation might happen. To get into details, you need to check
parameters A and B used in the prob model (they are available in the model
file)

olologin writes:

I know that it's weird usage of class weights, but stil, can it be explained
somehow? Or fixed?

dataset.txt:

0 1:0 2:0 3:0
0 1:0 2:0 3:1
0 1:0 2:1 3:0
1 1:0 2:1 3:1
1 1:1 2:0 3:0
1 1:1 2:0 3:1
2 1:1 2:1 3:0
2 1:1 2:1 3:1

code:

libsvm-3.20$ ./svm-train -b 1 -w0 1 -w1 1 -w2 0 dataset.txt model
libsvm-3.20$ ./svm-predict -b 1 dataset.txt model predictions.out

It produces in predictions.out:

labels 0 1 2
2 3.31221e-14 3.30357e-14 1
2 3.63995e-14 3.24543e-14 1
2 3.36039e-14 3.30595e-14 1
2 3.77311e-14 3.12876e-14 1
2 3.86737e-14 2.78238e-14 1
2 3.82377e-14 2.50579e-14 1
2 3.84825e-14 2.96375e-14 1
2 3.84239e-14 2.58019e-14 1

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from libsvm.

Thanks for explanation, do i need to close it? Because it seems that it's not issue, at least in mathematical terms.

from libsvm.

Yes, please.
olologin writes:

Thanks for explanation, do i need to close it? Because it seems that it's not
issue, at least in mathematical terms.

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That's contents of model which i used in first post:

svm_type c_svc
kernel_type rbf
gamma 0.333333
nr_class 3
total_sv 6
rho 0.0287883 -inf -inf
label 0 1 2
probA 1.44403 0 0
probB -0.010178 -0.287682 -0.287682
nr_sv 3 3 0
SV
1 0 1:0 2:0 3:0 
1 0 1:0 2:0 3:1 
1 0 1:0 2:1 3:0 
-1 0 1:0 2:1 3:1 
-1 0 1:1 2:0 3:0 
-1 0 1:1 2:0 3:1 

I have a question again:
After computing sigmoid with such probA and probB, from 2 classifiers we will get 0.5714... as probability of class 2, in any case, independently from X (because distance function is multiplied by 0). But how those one-vs-one probabilities are combined? I mean how SVM implementation achieves 1 from at least 2*0.5714? Simply sums probabilities for each class and divides by number of classes?

Thanks in advance.

from libsvm.

please check sec 8 of libsvm paper
http://www.csie.ntu.edu.tw/~cjlin/papers/libsvm.pdf
olologin writes:

That's contents of model which i used in first post:

svm_type c_svc
kernel_type rbf
gamma 0.333333
nr_class 3
total_sv 6
rho 0.0287883 -inf -inf
label 0 1 2
probA 1.44403 0 0
probB -0.010178 -0.287682 -0.287682
nr_sv 3 3 0
SV
1 0 1:0 2:0 3:0
1 0 1:0 2:0 3:1
1 0 1:0 2:1 3:0
-1 0 1:0 2:1 3:1
-1 0 1:1 2:0 3:0
-1 0 1:1 2:0 3:1

I have a question again:
After computing sigmoid with such probA and probB, from 2 classifiers we will
get 0.5714... as probability of class 2, in any case, independently from X
(because distance function is multiplied by 0). But how those one-vs-one
probabilities are combined? I mean how SVM implementation achieves 1 from at
least 2*0.5714? Simply sums probabilities for each class and divides by number
of classes?

Thanks in advance.

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from libsvm.