Bias Management in Time-Changing Data Streams

Model combination consists of creating a single learning system from a collection of learning algorithms. In some sense, model combination may be viewed as a variation on the theme of combining data mining operations discussed in Chapter 4. There are two basic approaches to model combination. The first one exploits variability in the application's data and combines multiple copies of a single learning algorithm applied to different subsets of that data. The second one exploits variability among learning algorithms and combines several learning algorithms applied to the same application's data.

The main motivation for combining models is to reduce the probability of misclassification based on any single induced model by increasing the system's area of expertise through combination. Indeed, one of the implicit assumptions of model selection in metalearning is that there exists an optimal learning algorithm for each task. Although this clearly holds in the sense that, given a task ø and a set of learning algorithms {

A

k

}, there is a learning algorithm

A

ø

in {

A

k

} that performs better than all of the others on ø, the actual performance of

A

ø

may still be poor. In some cases, one may mitigate the risk of settling for a suboptimal learning algorithm by replacing single model selection with model combination.