The reliability of the neural network
pattern recognition system is measured by testing the network with
hundreds of input vectors with varying quantities of
noise.
For example we create a noisy version (SD
0.2) of the letter ``J'' and query the network:
noisyJ = alphabet(:,10)+randn(35,1) * 0.2;
plotchar(noisyJ);
A2 = sim(net,noisyJ);
A2 = compet(A2);
answer = find(compet(A2) == 1);
plotchar(alphabet(:,answer));
Here is the noisy letter and the letter the
network picked (correctly).
Figure 3: The network is
tested with a noisy version of the letter ``J''.

The script file appcr1 tests the network at
various noise levels, and then graphs the percentage of network
errors versus noise. Noise with a mean of 0 and a standard
deviation from 0 to 0.5 is added to input vectors. At each noise
level, 100 presentations of different noisy versions of each letter
are made and the network's output is calculated. The output is then
passed through the competitive transfer function so that only one
of the 26 outputs (representing the letters of the alphabet), has a
value of 1.
The number of erroneous classifications is
then added and percentages are obtained.
Figure 4: Performance for
the network trained with and without noise.

The solid line on the graph shows the
reliability for the network trained with and without noise. The
reliability of the same network when it had only been trained
without noise is shown with a dashed line. Thus, training the
network on noisy input vectors greatly reduces its errors when it
has to classify noisy vectors.
The network did not make any errors for
vectors with noise of std 0.00 or 0.05. When noise of std 0.2 was
added to the vectors both networks began making errors.
If a higher accuracy is needed, the network
can be trained for a longer time or retrained with more neurons in
its hidden layer. Also, the resolution of the input vectors can be
increased to a 10by14 grid. Finally, the network could be trained
on input vectors with greater amounts of noise if greater
reliability were needed for higher levels of noise.
