28 Oct

learning how to learn

Yesterday’s attempts at ANN training seemed at first to be successful, but I had overlooked one simple put curious flaw – there was training going on all the time, and each test was run 10 times… This means that each neuron was trying different values all the time until it got the right one, then it would be the next neuron’s turn (a bit of a simplified answer, but I don’t know how to describe what was actually happening). This ended up causing the tests to look a lot more successful than they actually were.

This morning, I did a lot of work figuring out how to get around the problem. It turns out the problem is not with the neural network – that appears to be working perfectly. The problem is with the method of training.

Just like with people, you cannot just throw a net into a series of 26 tests which it has never seen before and expect it to learn it any time soon. For any particular neuron, 25 of the tests will have “No” as the answer, and it is too easy for the neuron to just answer “No” to everything and get a 96+% correct answer.

Instead, you need to start with just one test, keep trying until that’s right, then add another test, keep going until they’re both right, then add another test, etc.

Even that was not enough, though – it turns out that the capital letters “B” and “E” are very similar to each other, as are “H” and “M” (at least, in my sequence, they are).

I managed to improve the learning of the neurons by following rules such as these:

  • If a test’s answer is “No” but the neuron says “Yes” (ie; has a returned value above 0), then adjust the weights of the neuron it (correct/punish it).
  • If a test’s answer is “Yes”, and the neuron is anywhere less than 75% certain of Yes, then adjust/reward the neuron.

In all other cases (neuron is certain of Yes and is right, or neuron is vaguely sure of No and is right) leave the neuron’s weights alone.

This has helped to avoid the problem where neurons get extremely confident of Yes or No and are hard to correct when a similar test to a previously learned one comes along (O and Q for example).

It’s still not perfect, but perfection takes time…

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