Saturday, May 27, 2006

Neural Network craziness

Neural networks are a cool tool for what amounts to regression of data. In implementing a simple feed-foward, three-layer neural network and trying to get it to learn the majority function (are more than half of the inputs active?), I made a few interesting observations. I still have a lot to learn about neural networks, but in my novice jump into them, I've found that:

- The simple back-propagation algorithm is susceptible to bad starts, at least with a non-linear search space. When it does get off to a bad start, there's no turning back. So far, I've found that the best bet is to restart with random weights.

- In the back-propagation algorithm, adjusting the scalar variable (alpha) in an exponential decay manner (similar to simulated annealing) yields better results than keeping it constant.

- If the sum of the delta change in the weights is really large relatively speaking, then the network will likely suck.

- If the delta change in the weights keeps increasing, it will be difficult to turn back. The algorithm has probably fallen into a spiraling cycle.

- Having an imbalance of negative examples vs positive examples in a binary output variable for some reason makes the back propagation algorithm converge more quickly, at least in the majority problem.

- Even though the majority problem can be represented with a single hidden node (actually, just a two-layer network) with the bias being -n/2 and all weights being set to 1, the backprop algorithm as I have implemented it fails miserably with just one hidden node and a large number of inputs. It has greater success with several hidden node. This could just be an implementation issue, but my implementation follows text book examples pretty closely.

- Neural networks aren't all that hot for general classification problems, but they're a good complement at times.


Anonymous Brent O'Connor said...

Uh say what?? You lost me at, "back-propagation algorithm".

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