Intelligent Bayes Classifier (IBC) for ENT infection classification in hospital environment

7.1.1 MLP classifier

The most common neural network model is the multilayer perceptron (MLP). This type of neural network is known as a supervised network because it requires a desired output in order to learn. The goal of this type of network is to create a model that correctly maps the input to the output using historical data so that the model can then be used to produce the output when the desired output is unknown. A graphical representation of an MLP is shown below in Figure 6.

7.1.2 RBF classifier

These networks have a static Gaussian function as the nonlinearity for the hidden layer processing elements. The Gaussian function responds only to a small region of the input space around the Gaussian centered. The key to a successful implementation of these networks is to find suitable centres for the Gaussian functions. This can be done with supervised learning, but an unsupervised approach usually produces better results. For this reason, NeuroSolutions implements RBF networks as a hybrid supervised-unsupervised topology.

The simulation starts with the training of an unsupervised layer. Its function is to derive the Gaussian centres and the widths from the input data. These centres are encoded within the weights of the unsupervised layer using competitive learning. During the unsupervised learning, the widths of the Gaussians are computed based on the centres of their neighbors. The output of this layer is derived from the input data weighted by a Gaussian mixture.

Once the unsupervised layer has completed its training, the supervised segment then sets the centres of Gaussian functions (based on the weights of the unsupervised layer) and determines the width (standard deviation) of each Gaussian. Any supervised topology (such as a MLP) may be used for the classification of the weighted input.

The advantage of the radial basis function network is that it finds the input to output map using local approximators. Usually the supervised segment is simply a linear combination of the approximators. Since linear combiners have few weights, these networks train extremely fast and require fewer training samples. A graphical representation of an RBF is shown below in Figure 7.

7.1.3 PNN classifier

The PNN networks are variants of the radial basis function (RBF) network. Unlike the standard RBF, the weights of theses networks can be calculated analytically. In this case, the number of cluster centers is by definition equal to the number of exemplars, and they are all set to the same variance.

7.2.1 Basic idea: modeling of each class by a probability model

At the preliminary stage, we assign probability models to each of the classes. We assume that if we draw random samples from one class it will be selected from a fixed probability distribution, which is specific for that class. In general this means that the bacteria class has been modeled with that probability distribution model. So we assign a distribution to each of the classes. Assuming continuous probability models we specify a probability density function f(X/g) for class g.

7.2.3 Decision rule: maximum (Bayesian) probability rule

Baye's Rule: The posterior and prior probabilities of class membership are related using typicality probability of class by Baye's Rule in the following manner,

$P(g/{\text{X}}_{\text{u}})=\frac{{\pi }_g\cdot P({\text{X}}_{\text{u}}/g)}{{\displaystyle \sum _{g^{\prime }=1}^k{\pi }_{g^{\prime }}}\cdot P({\text{X}}_{\text{u}}/g^{\prime })}\left(3\right)$

So for classification purpose of a new unit of observation u to any one of the classes, our decision rule becomes:

Assign unit u to class g if P(g/X _u ) > P(h/X _u ) for g ≠ h.

This is called the "Maximum Probability Rule".

Again k values of P(X _u /g) need to be determined for each unit. Because the denominator in 1 is constant for all class, the rule could more simply be based on the k values of π _g ·f(X _u /g) and 1 can be stated equivalently as,

$P(g/{\text{X}}_{\text{u}})=\frac{{\pi }_g\cdot f({\text{X}}_{\text{u}}/g)}{{\displaystyle \sum _{g^{\prime }=1}^k{\pi }_{g^{\prime }}}\cdot f({\text{X}}_{\text{u}}/g^{\prime })}\left(4\right)$

7.2.4 Assignment of probability model

The "Maximum Probability Rule" involving posterior probability of class membership can be applied only if the probability density functions of the distributions of the classes are known [1, 29].

Each class may be modeled by assigning a density function f(X/g). Our only knowledge about class g is the N _g observation units of randomly drawn samples from class g. There are many commonly used techniques of estimating f(X/g) from this knowledge. Commonly these methods are divided in two classes, parametric and non-parametric ones.

Parametric approach: Specify a theoretical probability distributional model ${\text{f}}_{\underset{˜}{\theta }}$ (X/g), assume that the data on hand fit the model and estimate the model parameters $\underset{˜}{\theta }$ using the data, and construct a rule using these estimates.

Non-Parametric approach: Estimate the density values directly from data with no prior model specification, and construct a rule using these estimates.

We applied two significant methods from these two classes for classification of our ENT data. In parametric model multi-nominal model has been used, where in non-parametric approaches our choice was for "kernel density estimator". There are some other very successful non-parametric methods, like k-NN classification rule. It is true that the k-nn method is faster than kernel method. But where the minimization of classification error is also important then these two methods are equivalent when number of sensors is too large.

7.3.1 General kernel estimator based on Bayesian rule

In general the form of the kernel estimator is,

$\widehat{f}(X_u/g)=\frac{1}{N_g}{\displaystyle \sum _{i=1}^{N_g}K(X_u-X_i)}\left(5\right)$

Imposing the conditions K(z) ≥ 0 and ∫K(z)dz = 1 on K, then it is easy to see that $\widehat{f}$ also satisfies $\widehat{f}$ (z) ≥ 0 and ∫$\widehat{f}$ (z)dz = 1 so that $\widehat{f}$ is a legitimate pdf. Using product form of the kernel estimates we get,

$\widehat{f}(X_u/g)=\frac{1}{N_g}{\displaystyle \sum _{i=1}^{N_g}{\displaystyle \prod _{j=1}^p\frac{1}{h_{jg}}K(\frac{X_{uj}-X_{ij}}{h_{jg}})}}\left(6\right)$

Here the K is known as kernel function and ${\left\{h_{jg}\right\}}_{j=1,p}$ is called the smoothing parameters for the g-th class.

In our practice we use the normal kernel functions and take equal values for all ${\left\{h_{jg}\right\}}_{j=1,p}$ of g-th class, then our estimator becomes

$\widehat{f}(X_u/g)=\frac{1}{N_g}{\displaystyle \sum _{i=1}^{N_g}{\displaystyle \prod _{j=1}^p\frac{1}{h_g}}}\exp [-\frac{1}{2}(\frac{X_{uj}-X_{ij}}{h_g})]\left(7\right)$

Here we estimated h values for each class g and denoted them as h _g .

7.3.2 Adaptive kernel estimator as IBC

A practical drawback of the kernel method of density estimation is its inability to deal satisfactorily with the tails of distributions without over smoothing the main part of the density. The data have reasonable amount of outliers and their densities are multimodal densities. So the general method smoothed the inner part of the density, where it should not be due to the close overlapping of the data. This pattern of the data speaks for a density estimator which can sense small masses of probability and also a robust one at the same time. From the kernel density point of view, if the window function can be adopted locally depending upon the local data, we can have the optimum one. So here we used one of the popularly known adaptive approaches, adaptive kernel method. In this method another added local smoothing parameter has been used with the global smoothing parameter. This local one has been estimated from a pilot estimate of the density. Previous works show that this method tackles the tail probabilities excellently [1, 28–30].

The adaptive kernel estimators will be,

$\widehat{f}(X_u/g)=\frac{1}{N_g}{\displaystyle \sum _{i=1}^{N_g}\frac{1}{h_{ig}^{-p}}}K(X_u-X_i)\left(8\right)$

where the N _g smoothing parameters h _ig (i = 1,......., N _g ) are based on some pilot estimate of the density $\tilde{f}$ (X/g). The smoothing parameters h _ig are specified as h _g a _ig , where h _g is a global smoothing parameter of a class and a _ig are local smoothing parameters given by

$\begin{array}{cc}{a}_{ig}={\left\{\tilde{f}(X_{ig}/g)/C_g\right\}}^{-{\alpha }_g}& (i=1,\mathrm{.........},N_g)\end{array}\left(9\right)$

where $\log C_g=\frac{1}{N_g}{\displaystyle \sum _{i=1}^{N_g}\log (\tilde{f}(X_{ig}/g))}$

and $\tilde{f}$ (X _ig /g) > 0 (i = 1,........, N _g ), and where α _g is the sensitivity parameter satisfying 0 ≤ α _g ≤ 1. In practical application we assumed α _g to be 0.5.

Here in our case we have taken the general kernel estimator as the pilot estimate of the density $\tilde{f}$ (X/g).

Now plugging in these above estimates $\widehat{f}$ (X _u /g) in the equation 2 we get,

$\widehat{P}(g/{\text{X}}_{\text{u}})=\frac{q_g\cdot \widehat{f}(X_u/g)}{{\displaystyle \sum _{g^{\prime }=1}^k{q}_{g^{\prime }}\cdot \widehat{f}(X_u/g^{\prime })}}.\left(10\right)$

Then the decision rule becomes,

Assign unit u to class g if q _g ·$\widehat{f}$ (X _u /g) > q _h ·$\widehat{f}$ (X _u /h) for g ≠ h.

when $\widehat{f}$ (X _u /g) will be different for these two methods.

In both of these kernel methods we have to estimate some of the parameters, the window function h for each class. To simplify the problem we assumed same window function for each class, as because in kernel method this windows are smoothed again, so collective choice is not a bad idea. We selected the smoothing parameters simultaneously by minimization of the cross-validated estimate of the error of classification of the Baye's rule by plugging in these kernel estimates of the group-conditional densities from the knowledge.

It is pleasant to note that maximum probability rule is just a basic concept about modeling, and decision making rule. But it heavily depends upon the above described estimation step which is the heart of the modeling process. With the development of this estimation methods modeling becomes much more flawless and the classification technique becomes stronger. This phenomenon is noted at the time of testing. Adaptive kernel method with its superiority in estimation of tail probability gets an edge over the other methods.

8.1.1 Existing data manipulation method

Randomly we have taken out 20% of the feature points from the existing knowledge base and tried to classify them by considering the remaining 80% feature points in knowledge base as the "present knowledge base". In this way we have tried to classify all the data in the knowledge base and noted the percentage of correct classification for these different classes. Also we aimed to keep the consistency of this testing method along with ANN based testing methods. Advantage of this method is the tested data is virtually behaving like an unknown data. Following this approach we achieved 100% for the overall classification of three species of bacteria, Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and Pseudomonas aeruginosa (P. aeruginosa) responsible for ear nose and throat (ENT) infections.

In the next stage a sub-classification has been performed using IBC for the classification of two different species of S. aureus, namely MRSA and MSSA. In the later sub-classification problem we also achieved 100% rate of correct classification. All percentages of classification study are based on this our collected data sets at hospital.

8.1.2 Real data based method

To complete this study we decided to take a more realistic approach for testing. So far our classification performance testing was based on existing data sets which originally had been used to develop the knowledgebase. To make the validation of our IBC model in the hospital scenario, we gathered a completely new set of total 60 new patients' data who had infection from three species of bacteria, E. coli, S. aureus (including MRSA and MSSA), and P. aeruginosa.

Kurtosis and Skewness of this unknown data set were extracted as feature and the IBC was used to classify these species of bacteria along with the existing knowledgebase.

Results from these new set of data (completely unknown to our previously developed knowledgebase) were very encouraging. The best results suggest that we are able to identify and classify three bacteria main classes with up to 100% accuracy rate using IBC. We have also achieved 100% classification accuracy for the classification of MRSA and MSSA samples with IBC.