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I am not able to use a trained neural network ?

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Tonu
Tonu on 7 Mar 2014
Edited: Greg Heath on 1 May 2014
I have a set of data :
Input to the network for training : 2 inputs
input1 1e13 1.2e13 1.23e13 ..... input2 1e09 1.01e09 1.2e09 ......
ouput 1e11 2e11 2.2e11.......
I have such 31 data points and 31 target data points,
What, I have done is used the first 25 data points and trained and tested the network using nftool, remaing 6 data points I have used the trained network to predict and selected that network which gave me the least error in the output.
My question is :
(a) The trained network shows excellent match for the target data points but as soon as I put another set of data i.e. say input1 5e13 input2 2e09 , it gives me an output which does not make sense, i.e. gives me a value lesser than that which it had given when I was training the network i.e. say input1 4e13 input2 1.8e09 , it gave a value of say 4e11 , but for the former inputs it is giving me a value of 3.8e11 , which does not make any sense , How do I train the network better so that at least it follows the trend i.e. when input1 increases the output or the value thrown by the network should increase ?
(b) I even tried normalizing the data using premnmx, but not sure how do I use the trained network with a single value say input1 5e13 input2 2e09 i.e. How do I normalize this input, what shoudl be the reference to normalize the same ?
Your inputs would help tremendously,
Thanks, Tonu

Accepted Answer

Greg Heath
Greg Heath on 8 Mar 2014
Edited: Greg Heath on 8 Mar 2014
[ I N ] = size(x) % [ 2 31 ]
[O N ]= size(t)% [ 1 31]
1. For precision computing transform to zero mean and unit biased variance data (zx = zscore(x',1)', etc)
2. First time through use as many defaults as possible (trainlm with trn/val/tst =21/5/5 or trainbr with /25/0/6)
> What, I have done is used the first 25 data points and trained and tested > the network using nftool, remaing 6 data points I have used the trained > network to predict and selected that network which gave me the least > error in the output.
Very insufficient detail. Network type? No. of hidden nodes? No. of candidate designs with different initial weights? How did you avoid overfitting the network and memorizing training data?(val or trainbr?)
Many designs are needed to eliminate unfortunate values of initial weights and make sure that the network is not overfit (more weights than than the 26 training equations) and overtrained (memorizing training data without val or trainbr)).
First practice with MATLAB datasets to make sure you have enough data to obtain a good design.
Next use as many defaults as possible.
If no validation set use trainbr instead of trainlm.
If you are not using trainbr, try to minimize the number of hidden nodes.
Next time you post provide more details
Hope this helps.
Thank you for formally accepting my answer
Greg
  4 Comments
Greg Heath
Greg Heath on 1 May 2014
IT WOULD REALLY HELP IF YOU RAN YOUR CODE ON ONE OF THE EXAMPLE DATASETS IN
help nndatasets
doc nndatasets
>> net = fitnet(3) % NO SEMICOLON
%yields
performFcn: 'mse'
trainFcn: 'trainlm'
trainParam: .showWindow, .showCommandLine, .show, .epochs,
.time, .goal, .min_grad, .max_fail, .mu, .mu_dec,
.mu_inc, .mu_max
%Therefore lr and mc are not defined for the default trainlm.
% Since mse is the performance function
obj_fun = mse(o3-y_exp)
%Proper normalization by the variance of the target yields
normobj_fun = obj_fun/var(y_exp,1)
% The coefficient of determination, ( or R-squared See Wikipedia r-squared)is
R2 = 1-normobj_fun
% R2 > 0.99 means the net is sucessfully accounting for 99% of the target variance.

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