The objective of this study is to predict human wine taste preferences.
This can be useful to improve the wine production and to support the oenologist wine tasting evaluations.
Furthermore, similar techniques can help in target marketing by modeling consumer tastes from niche markets.
The variables used for this proposal are not related to grape type, wine brand or wine selling price, they are only related to physicochemical tests. The output of the model will give a score between 0 and 10, which defines the wine quality.
This is an approximation project, since the variable to be predicted is continuous (wine quality).
The basic goal here is to model the quality of a wine, as a function of its features.
The data file wine_quality.csv contains a total of 1599 rows and 12 columns. The first row in the data file contains the names of the variables and the rest of them represent the instances.
The data set contains the following variables:
On the other hand, the instances are divided at random into a training, a selection and a testing subsets, containing 60%, 20% and 20% of the instances, respectively.
Once the data set page has been edited we are ready to run a few related tasks. With that, we check that the provided information is of good quality.
We can calculate the data distributions and draws a histogram for each variable to see how they are distributed. The following figure shows the histogram for the quality data, which is the only target.
As we can see, the target variable is very unbalanced since there is a large amount of quality scores around 6 and much less with values near to 0 or 10. This will result into a poor quality model.
The next chart shows the correlations of the input variables with the target variable.
The second step is to set the neural network stuff. For approximation project types, it is usually composed by:
The scaling layer section contains the statistics on the inputs calculated from the data file and the method for scaling the input variables. Here the mean and standard deviation scaling method has been set. Nevertheless, the mean and standard deviation method would produce very similar results.
A hidden layer and a linear output layer of perceptrons will be used in this problem (this is the default in approximation). It must have 10 inputs, and 1 output neuron. While the numbers of inputs and output neurons are constrained by the problem, the number of neurons in the hidden layer is a design variable. Here we use 3 neurons in the hidden layer, which yields to 37 parameters. Finally, all the biases and synaptic weights in the neural network are initialized at random.
The unscaling layer contains the statistics on the outputs calculated from the data file and the method for unscaling the output variables. Here the minimum and maximum unscaling method will be used.
At last, in this example we will use a bounding layer, with 1 and 10 as the lower bound and upper bound respectively. The reason for this is that the value of the quality variable has clearly defined lower and upper limits.
The neural network for this example can be represented as the following diagram:
The function above is parameterized by all the biases and synaptic weights in the neural network, i.e, 37 parameters.
The next step is to select an appropriate training strategy, which defines what the neural network will learn. A general training strategy for approximation is composed of two terms:
The loss index chosen for this problem is the normalized squared error between the outputs from the neural network and the targets in the data set. On the other hand, L2 regularization with weak weight is applied here.
The selected optimization algorithm is the quasi-Newton method.
The most important training result is the final selection error. Indeed, this a measure of the generalization capabilities of the neural network. Here the final selection error is selection error = 0.678 NSE.
The objective of model selection is to find the network architecture with best generalization properties, that is, that which minimizes the error on the selection instances of the data set.
More specifically, we want to find a neural network with a selection error less than 0.678 NSE, which is the value that we have achieved so far.
Order selection algorithms train several network architectures with different number of neurons and select that with the smallest selection error.
The incremental order method starts with a small number of neurons and increases the complexity at each iteration. The following chart shows the training error (blue) and the selection error (orange) as a function of the number of neurons.
The final selection error achieved is 0.661 for an optimal number of neurons of 1.
The graph above represents the architecture of the final neural network.
A standard method for testing the prediction capabilities is to compare the outputs from the neural network against an independent set of data. The linear regression analysis, leads to 3 parameters to each output: intercept, slope and correlation. The next figure shows the results of this analysis.
If the correlation is equal to 1, then there is perfect correlation between the outputs from the neural network and the targets in the testing subset. As we can see for this case, the correlation has a value of R2 = 0.58, which indicates that the model is not predicting very well.
As it was stated previously, the quality data is remarkably unbalanced; this can be observed in the previous graph.
The model obtained after all the steps is not the best it could be achieved. Nevertheless, it is still better than guessing randomly.
The next listing shows the mathematical expression of the predictive model.
scaled_volatile_acidity = (volatile_acidity-0.527821)/0.17906; scaled_citric_acid = (citric_acid-0.270976)/0.194801; scaled_residual_sugar = (residual_sugar-2.53881)/1.40993; scaled_chlorides = (chlorides-0.0874665)/0.0470653; scaled_free_sulfur_dioxide = (free_sulfur_dioxide-15.8749)/10.4602; scaled_total_sulfur_dioxide = (total_sulfur_dioxide-46.4678)/32.8953; scaled_density = (density-0.996747)/0.00188733; scaled_pH = (pH-3.31111)/0.154386; scaled_sulphates = (sulphates-0.658149)/0.169507; scaled_alcohol = (alcohol-10.423)/1.06567; y_1_1 = tanh (-0.271612+ (scaled_volatile_acidity*-0.313035)+ (scaled_citric_acid*-0.124956)+ (scaled_residual_sugar*0.00194975)+ (scaled_chlorides*-0.0802881)+ (scaled_free_sulfur_dioxide*0.0392384)+ (scaled_total_sulfur_dioxide*-0.111936)+ (scaled_density*0.0456105)+ (scaled_pH*-0.11043)+ (scaled_sulphates*0.122299)+ (scaled_alcohol*0.361693)); scaled_quality = (0.159294+ (y_1_1*0.453138)); quality = (0.5*(scaled_quality+1.0)*(8-3)+3); quality = max(1, quality) quality = min(10, quality)
The formula from below can be exported to the software tool required by the customer.