quarta-feira, fevereiro 24, 2016

THE MACHINE LEARNING PROCESS

Data Collection and Preparation
Throughout this book we will be in the fortunate position of having datasets readily available for downloading and using to test the algorithms. This is, of course, less commonly the case when the desire is to learn about some new problem, when either the data has to be collected from scratch, or at the very least, assembled and prepared. In fact, if the problem is completely new, so that appropriate data can be chosen, then this process should be merged with the next step of feature selection, so that only the required data is collected. This can typically be done by assembling a reasonably small dataset with all of the features that you believe might be useful, and experimenting with it before choosing the best features and collecting and analysing the full dataset.
Often the difficulty is that there is a large amount of data that might be relevant, but it is hard to collect, either because it requires many measurements to be taken, or because they are in a variety of places and formats, and merging it appropriately is difficult, as is ensuring that it is clean; that is, it does not have significant errors, missing data, etc.
For supervised learning, target data is also needed, which can require the involvement of experts in the relevant field and significant investments of time.
Finally, the quantity of data needs to be considered. Machine learning algorithms need significant amounts of data, preferably without too much noise, but with increased dataset size comes increased computational costs, and the sweet spot at which there is enough data without excessive computational overhead is generally impossible to predict.

Feature Selection
It consists of identifying the features that are most useful for the problem under examination. This invariably requires prior knowledge of the problem and the data; our common sense was used in the coins example above to identify some potentially useful features and to exclude others.
As well as the identification of features that are useful for the learner, it is also necessary that the features can be collected without significant expense or time, and that they are robust to noise and other corruption of the data that may arise in the collection process.

Algorithm Choice
Given the dataset, the choice of an appropriate algorithm (or algo- rithms) is what this book should be able to prepare you for, in that the knowledge of the underlying principles of each algorithm and examples of their use is precisely what is required for this.

Parameter and Model Selection 
For many of the algorithms there are parameters that have to be set manually, or that require experimentation to identify appropriate values.

Training
Given the dataset, algorithm, and parameters, training should be simply the use of computational resources in order to build a model of the data in order to predict the outputs on new data.

Evaluation 
Before a system can be deployed it needs to be tested and evaluated for ac- curacy on data that it was not trained on. This can often include a comparison with human experts in the field, and the selection of appropriate metrics for this comparison.

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