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Back Propagation. Amir Ali Hooshmandan Mehran Najafi Mohamad Ali Honarpisheh. Contents. What is it? History Architecture Activation Function Learnig Algorithm EBP Heuristics How Long to Train Virtues AND Limitations of BP About Initialization Accelerating training An Application

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back propagation

Back Propagation

Amir Ali Hooshmandan

Mehran Najafi

Mohamad Ali Honarpisheh

  • What is it?
  • History
  • Architecture
  • Activation Function
  • Learnig Algorithm
  • EBP Heuristics
  • How Long to Train
  • Virtues AND Limitations of BP
  • About Initialization
  • Accelerating training
  • An Application
  • Different Problems Require Different Learning Rate Adaptive Methods
what is it
What is it
  • A Supervised learning algorithm
  • Base  Error correction learning rule
  • Generalize  Adaptive Filtering Algorithm
  • 1986
    • Rumelhart
      • Paper  Why are “what” and “where” processed by separate cortical visual systems?
      • Book  Parallel Distributed Processing: Explorations in Micro Structures of cognition
    • Parker
      • Optimal algorithms for adaptive networks:

second order back propagation

second order direct propagation

  • 1974 & 1969









activation function
Activation Function


output of neuron J

learnig algorithm
Learnig Algorithm

ek(n) = dk(n) - yk(n)

Energy in this error

Total energy of the

output of net:

Energy of the error

produced by output

Neuron J in epoch n

learnig algorithm cont d
Learnig Algorithm(cont.’d)


Minimizing E(n)

e_yk(n) = dk(n) - yk(n)

Local Field

learnig algorithm cont d1
Learnig Algorithm(cont.’d)

Chain Rule in Derivation


Local gradient

computing v i j for none output layers



Computing Δvi,j for none output layers


We don’t have error because

it is responsible for many errors

Find another way to compute δj

computing weight correction
Computing Weight correction

(Weight correction) = (Learning rate parameter)

* (local gradient)

* (input signal of previouse layer Neuron)

training algorithm
Training Algorithm

Step 0 : Initialize weights

(Set to random variables with zero mean and variance one)

Step 1: While stopping condition is false do Step 2-9.

Step 2:

For each training pair do Steps 3-8.

Feed forward

Step 3: Each input unit(Xi,i=1,..,n) receives

input signal xi and broadcasts this signal

to all units in the layer above(the hidden units(

training algorithm cont
Training Algorithm(Cont)

Step 4: Each hidden unit (Zj j=1,…,p) sums its weighted input signals

applies its activation function to compute its output signal

and sends this signal to all units in the layer above


training algorithm cont1
Training Algorithm(Cont)

Step 5: Each output unit (Yk ,k=1,…..,m) sums its weighted input signals,


and applies its activation function to compute its output signal.


training algorithm cont2
Training Algorithm(Cont)

Backpropagation of error:

Step 6: Each output unit ( Yk ,k=1,…,m) receives a target

pattern corresponding to the input training patern

computes its error information term.

calculates its weight correction term (used to update wjk later),

calculates its bias correction term ( used to update wOk later).

and sends to units in the layer below,

training algorithm cont3
Training Algorithm(Cont)

Step 7: Each hidden units (Zj, j=1,…,p) sums its delta inputs from units in the layer above).


multiplies by the derivative of its activation function to

calculate its error information term,

calculates its weight correction term(used to update vij later),

and calculates its bias correction term(used to update voj later),

training algorithm cont4
Training Algorithm(Cont)

Update weights and biases:

Step 8: Each output units(Yk,k=1,….,m) updates its bias

and weights(j=0,…,p):


Each hidden unit(Zj j=1,….,p) updates its bias and



Step 9: Test stopping condition

ebp heuristics
EBP Heuristics
  • Number Of Hidden Layers :
    • Theoretical & Simulation results showed that there is no need to have more than two hidden layers.
    • One OR Two hidden layers…?
      • Chester ( 1990 ) : “Why two hidden layers are better than one”
      • Gallant ( 1990 ) : “Never try a multilayer model for fitting

data until you have first tried a single-layer model”

Both architectures are theoretically able to approximate any continuous function to the desired degree of accuracy.

ebp heuristics cont d
EBP Heuristics (cont’d)
  • Number of hidden layers… :
    • Its difficult to say which topology is better :
      • Size of NN
      • Learning Time
      • Implementability in hardware
      • Accuracy
    • Solving problem using NN with one hidden layer first , seems appropriate.
ebp heuristics cont d1
EBP Heuristics (cont’d)
  • Every adjustable network parameter of the cost function should have its own individual learning rate parameter.
  • Every Learning rate parameter should be allowed to vary from one iteration to the next.
  • Increasing weight ‘s LR that has same derivative sign for several iterations.
  • Decreasing weight’s LR that has alternating derivative sign.
how long to train
How Long to Train
  • The Aim is to balance between Generalization & Memorization

( Minimizing cost function is not necessarily good idea ).

    • Hecht-Nielsen( 1990 ) : Using two disjoint sets for training
      • Training Set
      • Training-Testing Set
    • As long as the error for the training-testing set decreases , training continues.
    • When the error begin to increase , the net is starting to memorize.
virtues and limitations of bp
Virtues AND Limitations of BP
  • Connectionism
    • Biological Reasons
      • No excitatory or inhibitory for real neurons
      • No Global connection in MLP
      • No backward propagation in real neurons
    • Useful in parallel hardware implementation
    • Fault Tolerance
virtues cont d
Virtues…( Cont’d )
  • Computational Efficiency
    • Computational complexity of alg. is measured in terms of multiplications,additions…
    • Learning Algorithm is said to be computationally efficient , when its complexity is polynomial…
    • The BP algorithm is computationally efficient.
      • In MLP with a total W weights , its complexity is linear in W
virtues cont d1
  • Convergence

Saarinen (1992 ) : Local convergence rates of the BP algorithm are linear

    • Too flat OR too curved
    • Wrong Direction
  • Local Minima
    • BP learning is basically a Hill climbing technique.
    • Presence of local minima( isolated valleys )
about init cont
About Init… (cont)
  • Other issues :
    • Initialization of OL weights shouldn’t result in small weighs...?
      • If the output layer weights are small, then so is the

contribution of the HL neurons to the output error, and consequently the effect of the hidden layer weights is not visible enough.

      • If the OL weights are too small, deltas( for HLs ) also become very small, which in turn leads to small initial changes in the hidden layer weights.
about init cont1
About Init… (cont)
  • Initialization by using random numbers is very important in avoiding the effects of symmetry in the network. all the HL neurons should start with guaranteed different weight.
    • If they have similar (or, even worse, the same) weights, they will perform similarly (the same) on all data pairs by changing weights in similar (the same) directions.
    • This makes the whole learning process unnecessarily long (or learning will be the same for all neurons, and there will practically be no learning).
nguyen widrow initialization
Nguyen – Widrow Initialization…
  • Two Layer NNs have been proven capable of approximating any arbitrary functions…
    • How this work?
    • and method for speeding up training process…
behavior of hidden nodes
Behavior of hidden nodes…
  • For simplicity two layer network with one input is trained to approximate a function of one variable d(x). “x” as input and using BP algorithm…
    • Output is in the form of :
    • Sigmoid function ( tanh ) :
      • Approximately linear with slope 1 for x between -1 and 1.
      • Saturate to -1 or 1 as x becomes large in magnitude
      • Each term in above some is linear function of x over small interval

Size of each interval is determined by wi

  • Location of interval is determined by wbi
  • Network learns to implement desired function by building piece-wise linear approximations
  • Pieces are summed to form the complete approximation
  • (Random Initialization)
improving learning speed
Improving Learning Speed
  • Main Idea :
    • Divide desired region into small intervals
    • Setting weights in a manner that each hidden node is assigned to its own interval at start of training.
    • Training is as before…
improving cont
Improving … (cont)
  • Desired region : (-1,1) , has length 2
  • H hidden units
    • So, each hidden unit is responsible for an interval of length 2/H
    • Sigmoid(wi x + wbi ) is approximately linear over :
    • Which has length 2/wi , therefore :
    • Its preferable to have intervals overlap :
    • For wbi :

Training of a net initialized as discussed…

Net whose weights are initialized to random values between -.5 and .5

  • Improvement is best when a large number of hidden units is used with a complicated desired response.
  • Training time decreased from 2 days to 4 hours for Truck-Backer-Upper
  • Weight change’s Direction : Combination of current gradient and previous gradient.
    • Advantage : Reduce the role of outliers
    • Doesn’t adjust LR directly.

Momentum Parameter , its in the range from 0 to 1

momentum cont d
Momentum (cont’d)
  • Allows large weight adjustments as long as the correction is in the same direction…
  • Forms an exponentially weighted sum :
  • BP Vs MOM : XOR function with bipolar representation
delta bar delta
  • Allows each weight to have its own learning rate
  • Lets learning rates vary with time
  • Two heuristics are used to determine appropriate changes :
    • If weight changes is in the same direction for several time steps , LR for that weight should be increased.
    • If direction of weight change alternates , the LR should be decreased.
  • Note : these heuristics wont always improve the performance.
dbd cont d
DBD (cont’d)
  • DBD rule consists of :
    • Weight update rule
    • LR update rule
  • DBD rule changes the weights as follows :

Use information of current and past derivative to form “delta-bar”

dbd cont d1
DBD (cont’d)
  • The 1st heuristic is implemented by increasing the LR by a constant amount :
  • 2nd heuristic is implemented by decreasing LR by a proportion of its current value :
  • LR increase linearly and decrease exponentially.
dbd cont d2
DBD (cont’d)

Results for XOR …

  • neural network (NN) techniques can be used in detecting intruders logging on to a computer network.
  • compares the performance of the four neural network methods in intrusion detection.
  • The NN techniques used are

1. gradient descent back propagation (BP)

2. gradient descent BP with momentum

3. variable learning rate gradient descent BP

4. Conjugate Gradient BP (CGP)

background on intrusion detection systems ids
Background on Intrusion Detection Systems (IDS)
  • Information assurance is a field that deals with protecting information on computers or computer networks from being compromised.
  • Intrusion detection :

detecting unauthorized users from accessing the information on those computers.

  • Current intrusion detection techniques can not detect new and novel attacks
  • The relevance of NN in intrusion detection becomes apparent when one views the intrusion detection problem as a pattern classification problem.
pattern classification problem
Pattern Classification Problem
  • By building profiles of authorized computer users one can train the NN to classify the incoming computer traffic into authorizedtraffic or not authorized traffic.
  • The task of intrusion detection is to construct a model that captures a set of user attributes and determine if that user’s set of attributes belongs to the authorized user or those of the intruder.
problem definition
Problem Definition
  • Attribute set consists of the unique characteristics of the user logging onto a computer network:
    • Authorized user and Intruder
  • The problem can be stated as:
  • Where:

x = input vector consisting a user’s attributes

y = {authorized user, intruder}

We want to map the input set x to an output

solving the intrusion detection problem using the back propagation
Solving the Intrusion DetectionProblem Using The BackPropagation
  • Multilayer Perceptron with Two Hidden Layers

The error of our model is: e = d - y

d = desired output

y = actual output

  • Activation functions

Sigmoidal :

  • That users in the UNIX OS environment could be profiled via four attributes

command, host, time, execution time

  • For simplicity in testing the back propagation methods, we decided to generate a user profile data file without profile drift
  • The generated data used here was organized into two parts.

Training Data :

90% Authorized traffic

10% Intrusion traffic

Testing Data:

98% Authorized traffic

2% Intrusion traffic

  • Our objective is to train the neural networks for detecting intrusion traffic with the fewest number of intrusion samples

File1 consists of 5 CUs in each of the input

File2 consists of 6 CUs in each of the input

File3 consists of 7 CUs in each of the input

Each CU has 4 elements

  • We investigate three kind of Error Back Propagation Neural Network and result’s
gradient descent bp gd
Gradient Descent BP (GD)
  • This method updates the network weights and biases in the direction of the performance function that decreases most rapidly, i.e. the negative of the gradient. The new weight vector wk+1 is adjusted according to:
  • α is the learning rate and gk is the gradient of the error with respect to the weight vector
  • The negative sign indicates that the new
  • Weight vector wk+1 is moving in a direction opposite to that of the gradient.
gradient descent bp with momentum gdm
Gradient Descent BP with Momentum (GDM)
  • Making weight changes equal to the sum of a fraction of the last weight change and the new change suggested by the gradient descent BP rule.
  • Advantage:

1. Momentum allows a network to respond not only to the local gradient, but also to recent trends in the error Surface

2. Momentum allows the network to ignore small features in the error surface.

3. Without momentum a network may get stuck in shallow local minimum. With momentum a network can slide through such a minimum

gradient descent bp with momentum gdm1
Gradient Descent BP with Momentum (GDM)
  • µ, which can be any number between 0 and 1.
variable learning rate bp with momentum gdx
Variable Learning Rate BP with Momentum (GDX)
  • The learning rate parameter is used to determine how fast the BP method converges to the minimum solution.
  • If the learning rate is made too large :

the algorithm will become unstable.

  • if the learning rate is set to too small :

the algorithm will take a long time to converge.

  • To speed up the convergence time, the variable learning rate gradient descent BP utilizes larger learning rate α when the neural network model is far from the solution and smaller learning rate α when the neural net is near the solution.
variable learning rate bp with momentum gdx1
Variable Learning Rate BP with Momentum (GDX)
  • The new weight vector wk+1 is adjusted the same as in the gradient descent with momentum above but with a varying αk
conjugate gradient bp cgp
Conjugate Gradient BP (CGP)
  • In the conjugate gradient algorithms a search is performed along

conjugate directions, which produces generally faster convergence

Than steepest descent directions.

  • A search is made along the conjugate gradient direction to determine the step size which will minimize the performance function along that line
performance comparison
Performance Comparison
  • Result when Input is File1
performance comparison1
Performance Comparison
  • Result when Input is File2
performance comparison2
Performance Comparison
  • Result when Input is File3
results from tables
Results From Tables
  • 1.the gradient descent with momentum method was not as good in detecting the intrusion traffic from the authorized traffic as the gradient descent method.
  • 2.gradient descent with momentum methods were able to classify intrusion traffic from authorized traffic but with higher MSE values.
  • 3. the number of samples used as inputs affected the performance of the classification of the data

Output Values of the Two Classes

results from tables1
Results From Tables
  • The gradient descent, the gradient descent with momentum and the variable rate gradient descent with momentum method converged to a MSE = exp(-3). they were able to classify the intrusion traffic from the authorized traffic.
  • The input sample that yielded the best performance for all five methods contained 6 CUs
  • The number of neurons used at the hidden layer depended on the number of CUs in the input samples. In these cases, the NN topology of {24,10,1} yielded the best results.
  • When the input data files were File1 and File3 (i.e. input sample consists of 5 or 7 CUs), we did not get good results compare to the case when the input is File2

(Memorize & Generalize Problem)

  • Fourth, the conjugate gradient descent have the best performance
general parameter settings

General parameter settings

Network Architecture : standard feedforward neural network.

Maximum of parameters was constant for all tasks.

AFs for non-input units : standard hyperbolic tangent.

Correctness : All output units produce correct answer.

Seven networks for each configuration…

Fixed total number of itteration.

different problems
Different Problems
  • Each algorithm tested on three different tasks
    • Parity Bit : # of activated input units is odd or even. In this study input layer is composed of nine units each has two states , 512 different patterns. Only one output layer is needed.
    • N-M-N Encoder : The encoder task consisted of reproducing the same output activation pattern as the input one. In the activation patterns , one unit is active and the others aren’t. The complexity of this task resided in the fact that the number of hidden units was less than the number of input and output units (M<N).[M=7 , N=16].
  • The task consisted in detecting the orientation, either horizontal or vertical, of stripes defined by texture in an image.
algorithm dependent parameter settings
Algorithm dependent parameter settings
  • The main problem with AMs is that they require many parameters that are, like the LR, problem dependent. It is impossible to test all possible parameter combinations. Usually, to compare AMs for a given task, authors affirm that they tried to find “good” parameter settings! ( easiness of finding these params …. )
    • For MOM , the free parameter is momentum factor. Tested values are : 0 , .5 , .7 , .9 and .99 .
    • For DBD , the other parameter is increase factor.
results for parity bit
Results for Parity-Bit

Free Parameters


Each element represents the number of times the AM solve the task (max=7) for a given parameter combination. The lines correspond to different LRs and the columns to different free parameter values.

MOM and DBD achieving a performance of 6/7 .

results for encoder
Results For Encoder

For DBD and MOM, although many parameter combinations solved the task, none resulted in a one hundred percent efficacy.

results for texture
Results for Texture
  • No parameter combination was found that could solve the texture task using MOM.
  • DBD solved the task only if the initial LR was 4^(-2).
  • With the proper initial LR the other DBD parameter (incremental constant) showed greater flexibility
  • The first and obvious conclusion that can be drawn from these results is that no AM could attain a better performance than all the others on all tasks
  • MOM and DBD had a similar behavior when they were used on the encoder and parity task.
  • The only task of which they clearly differed was the texture task where MOM never solved the task as opposed to DBD.
comparison of nn svm
Comparison Of NN & SVM
  • Problem : Recognizing Young-Old Gait Patterns
    • The gaits of 12 young and 12 elderly participants were recorded and analyzed.
    • 24 gait parameters (features) were extracted for training and testing the NN and SVM systems.
    • NNs have been employed to classify normal and pathological gait… with good success.
    • SVMs have emerged as a new and powerful technique for learning from data and in particular for solving classification and regression problems with reported better performance.
gate parameters
Gate Parameters
  • Three types of gait params :
    • Basic Gate Data ( 9 Variables ) : Walking speed , stride length , ….
    • Kinetic Data ( 5 Variables ) : Foot-Ground forces ….
    • Kinematic Data( 10 Variables ) : knee and ankle joint angles … .
experimental results
Experimental Results
  • A total of 24 subjects were used , which 20 subjects’ data were used to train the NN and SVM, and the remaining 4 subjects were used to test the generalization ability of both techniques.
  • Due to small sample size, a cross-validation technique was employed. In this way , all 24 subjects appeared in the testing phase of the NN and SVM models.( 6 group , 4 in each one )
  • Each Alg. Is tested 20 times.