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Artificial Intelligence 13. Multi-Layer ANNs Course V231 Department of Computing Imperial College © Simon Colton

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Multi-Layer Networks Built from Perceptron Units Perceptrons not able to learn certain concepts – Can only learn linearly separable functions But they can be the basis for larger structures – Which can learn more sophisticated concepts – Say that the networks have “perceptron units”

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Problem With Perceptron Units The learning rule relies on differential calculus – Finding minima by differentiating, etc. Step functions aren’t differentiable – They are not continuous at the threshold Alternative threshold function sought – Must be differentiable – Must be similar to step function i.e., exhibit a threshold so that units can “fire” or not fire Sigmoid units used for backpropagation – There are other alternatives that are often used

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Sigmoid Units Take in weighted sum of inputs, S and output: Advantages: – Looks very similar to the step function – Is differentiable – Derivative easily expressible in terms of σ itself:

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Example ANN with Sigmoid Units Feed forward network – Feed inputs in on the left, propagate numbers forward Suppose we have this ANN – With weights set arbitrarily

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Propagation of Example Suppose input to ANN is 10, 30, 20 First calculate weighted sums to hidden layer: – S H1 = (0.2*10) + (-0.1*30) + (0.4*20) = = 7 – S H2 = (0.7*10) + (-1.2*30) + (1.2*20) = = -5 Next calculate the output from the hidden layer: – Using: σ(S) = 1/(1 + e -S ) – σ(S H1 ) = 1/(1 + e -7 ) = 1/( ) = – σ(S H2 ) = 1/(1 + e 5 ) = 1/( ) = – So, H1 has fired, H2 has not

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Propagation of Example Next calculate the weighted sums into the output layer: – S O1 = (1.1 * 0.999) + (0.1 * ) = – S O2 = (3.1 * 0.999) + (1.17 * ) = Finally, calculate the output from the ANN – σ(S O1 ) = 1/(1+e ) = 1/( ) = – σ(S O2 ) = 1/(1+e ) = 1/( ) = Output from O2 > output from O1 – So, the ANN predicts category associated with O2 – For the example input (10,30,20)

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Backpropagation Learning Algorithm Same task as in perceptrons – Learn a multi-layer ANN to correctly categorise unseen examples – We’ll concentrate on ANNs with one hidden layer Overview of the routine – Fix architecture and sigmoid units within architecture i.e., number of units in hidden layer; the way the input units represent example; the way the output units categorises examples – Randomly assign weights to the the whole network Use small values (between –0.5 and 0.5) – Use each example in the set to retrain the weights – Have multiple epochs (iterations through training set) Until some termination condition is met (not necessarily 100% acc)

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Weight Training Calculations (Overview) Use notation w ij to specify: – Weight between unit i and unit j Look at the calculation with respect to example E Going to calculate a value Δ ij for each w ij – And add Δ ij on to w ij Do this by calculating error terms for each unit The error term for output units is found – And then this information is used to calculate the error terms for the hidden units So, the error is propagated back through the ANN

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Propagate E through the Network Feed E through the network (as in example above) Record the target and observed values for example E – i.e., determine weighted sum from hidden units, do sigmoid calc – Let t i (E) be the target values for output unit i – Let o i (E) be the observed value for output unit i Note that for categorisation learning tasks, – Each t i (E) will be 0, except for a single t j (E), which will be 1 – But o i (E) will be a real valued number between 0 and 1 Also record the outputs from the hidden units – Let h i (E) be the output from hidden unit i

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Error terms for each unit The Error Term for output unit k is calculated as: The Error Term for hidden unit k is: In English: – For hidden unit h, add together all the errors for the output units, multiplied by the appropriate weight. – Then multiply this sum by h k (E)(1 – h k (E))

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Final Calculations Choose a learning rate, η (= 0.1 again, perhaps) For each weight w ij – Between input unit i and hidden unit j – Calculate: – Where x i is the input to the system to input unit i for E For each weight w ij – Between hidden unit i and output unit j – Calculate: – Where h i (E) is the output from hidden unit i for E Finally, add on each Δ ij on to w ij

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Worked Backpropagation Example Start with the previous ANN We will retrain the weights – In the light of example E = (10,30,20) – Stipulate that E should have been categorised as O1 – Will use a learning rate of η = 0.1

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Previous Calculations Need the calculations from when we propagated E through the ANN: t 1 (E) = 1 and t 2 (E) = 0 [from categorisation] o 1 (E) = and o 2 (E) = 0.957

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Error Values for Output Units t 1 (E) = 1 and t 2 (E) = 0 [from categorisation] o 1 (E) = and o 2 (E) = So:

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Error Values for Hidden Units δ O1 = and δ O2 = h 1 (E) = and h 2 (E) = So, for H1, we add together: – (w 11 *δ 01 ) + (w 12 *δ O2 ) = (1.1*0.0469)+(3.1* ) = – And multiply by: h 1 (E)(1-h 1 (E)) to give us: * (0.999 * ( )) = = δ H1 For H2, we add together: – (w 21 *δ 01 ) + (w 22 *δ O2 ) = (0.1*0.0469)+(1.17* ) = – And multiply by: h 2 (E)(1-h 2 (E)) to give us: * (0.067 * ( )) = = δ H2

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Calculation of Weight Changes For weights between the input and hidden layer

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Calculation of Weight Changes For weights between hidden and output layer Weight changes are not very large – Small differences in weights can make big differences in calculations – But it might be a good idea to increase η

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Calculation of Network Error Could calculate Network error as – Proportion of mis-categorised examples But there are multiple output units, with numerical output – So we use a more sophisticated measure: Not as complicated as it looks – Square the difference between target and observed Squaring ensures we get a positive number Add up all the squared differences – For every output unit and every example in training set

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Problems with Local Minima Backpropagation is gradient descent search – Where the height of the hills is determined by error – But there are many dimensions to the space One for each weight in the network Therefore backpropagation – Can find its ways into local minima One partial solution: – Random re-start: learn lots of networks Starting with different random weight settings – Can take best network – Or can set up a “committee” of networks to categorise examples Another partial solution: Momentum

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Adding Momentum Imagine rolling a ball down a hill Without Momentum With Momentum Gets stuck here

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Momentum in Backpropagation For each weight – Remember what was added in the previous epoch In the current epoch – Add on a small amount of the previous Δ The amount is determined by – The momentum parameter, denoted α – α is taken to be between 0 and 1

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How Momentum Works If direction of the weight doesn’t change – Then the movement of search gets bigger – The amount of additional extra is compounded in each epoch – May mean that narrow local minima are avoided – May also mean that the convergence rate speeds up Caution: – May not have enough momentum to get out of local minima – Also, too much momentum might carry search Back out of the global minimum, into a local minimum

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Problems with Overfitting Plot training example error versus test example error: Test set error is increasing! – Network is overfitting the data – Learning idiosyncrasies in data, not general principles – Big problem in Machine Learning (ANNs in particular)

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Avoiding Overfitting Bad idea to use training set accuracy to terminate One alternative: Use a validation set – Hold back some of the training set during training – Like a miniature test set (not used to train weights at all) – If the validation set error stops decreasing, but the training set error continues decreasing Then it’s likely that overfitting has started to occur, so stop – Be careful, because validation set error could get into a local minima itself Worthwhile running the training for longer, and wait and see Another alternative: use a weight decay factor – Take a small amount off every weight after each epoch – Networks with smaller weights aren’t as highly fine tuned (overfit)

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Suitable Problems for ANNs Examples and target categorisation – Can be expressed as real values ANNs are just fancy numerical functions Predictive accuracy is more important – Than understanding what the machine has learned Black box non-symbolic approach, not easy to digest Slow training times are OK – Can take hours and days to train networks Execution of learned function must be quick – Learned networks can categorise very quickly Very useful in time critical situations (is that a tank, car or old lady?) Error: ANNs are fairly robust to noise in data

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