features mean later. Each line of the table is one of the examples. Part of a struc-
tural description of this information might be as follows:
If tear production rate =reduced then recommendation =none
Otherwise, if age =young and astigmatic = no
then recommendation =soft
Structural descriptions need not necessarily be couched as rules such as these.
Decision trees, which specify the sequences of decisions that need to be made
and the resulting recommendation, are another popular means of expression.
This example is a very simplistic one. First, all combinations of possible
values are represented in the table. There are 24 rows, representing three possi-
ble values of age and two values each for spectacle prescription, astigmatism,
and tear production rate (3 ¥ 2 ¥ 2 ¥ 2 =24). The rules do not really general-
ize from the data; they merely summarize it. In most learning situations, the set
of examples given as input is far from complete, and part of the job is to gen-
eralize to other, new examples. You can imagine omitting some of the rows in
the table for which tear production rate is reducedand still coming up with the
rule
If tear production rate =reduced then recommendation =nonewhich would generalize to the missing rows and fill them in correctly. Second,
values are specified for all the features in all the examples. Real-life datasets
invariably contain examples in which the values of some features, for some
reason or other, are unknown—for example, measurements were not taken or
were lost. Third, the preceding rules classify the examples correctly, whereas
often, because of errors or noisein the data, misclassifications occur even on the
data that is used to train the classifier.
Machine learning
Now that we have some idea about the inputs and outputs, let’s turn to machine
learning. What is learning, anyway? What is machine learning? These are philo-
sophic questions, and we will not be much concerned with philosophy in this
book; our emphasis is firmly on the practical. However, it is worth spending a
few moments at the outset on fundamental issues, just to see how tricky they
are, before rolling up our sleeves and looking at machine learning in practice.
Our dictionary defines “to learn” as follows:
To get knowledge of by study, experience, or being taught;
To become aware by information or from observation;
To commit to memory;
To be informed of, ascertain;
To receive instruction.1.1 DATA MINING AND MACHINE LEARNING 7