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I use a MLP with one hidden layer (15 nodes) and one output node. I use a sigmoid activation function, atan as error function, the error itself is calculated with MSE, 5-fold crossvalidation, resilient backpropagation for a batched binary classification task where within each batch approx. 1000 samples are available.

My original dataset has a ratio of approx. 30/70 positive vs. negative samples. No matter what NN setup I tried (more features, more samples) the training error didn't go beneath 0.1, the f-measure I used for evaluation was between 0.3-05, precision 0.6-0.8 and recall only between 0.2-0.4.

Then I tried oversampling in order to increase the positive/negative value to approx. 1. Now with the same setup I the error decreased only to 0.09, but now I get a constant f-measure of > 0.85 , precision around 0.8 and recall 0.95-1(!?).

Now I'm really wondering if my setup is completely wrong or if have found a way to fit my data well.

Does anybody have some hints where I might have made a mistake or do you think my setup is ok and my classifier, too?

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  • $\begingroup$ Have you tried adding skip-layer connections? I usually run skip connections with a weight decay (aka ridge penalty). If all the hidden -> output connections get shrunk to zero, then I tend to believe the interactions are weak sources of information for a given data set... $\endgroup$ – Shea Parkes Feb 2 '12 at 21:26
  • $\begingroup$ I removed the spoiling feature. Without oversampling now my error is constantly at 0.2 and the f-measure around 0.2-0.4. With oversampling the error stays almost the same, but the f-measure goes up to 0.7-0.8. So now back to my original question: Is my setup and the oversampling approach completely wrong? $\endgroup$ – Andreas Feb 6 '12 at 6:37
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I found a spoiling feature which caused a linear dependency between input and output. That's why my upsampling increased the measurement values so good.

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