One may be interested in obtaining an estimate for any
of the many performance indicators such as accuracy,
precision, recall, or F-score. Cross-validation allows for
all the data to be used in obtaining an estimate. Most
commonly one wishes to estimate the accuracy of a
classifier in a supervised-learning environment. In
such a setting, a certain amount of labeled data is
available and one wishes to predict how well a certain
classifier would perform if the available data is used to
train the classifier and subsequently ask it to label
unseen data. Using 10-fold cross-validation one repeatedly
uses 90% of the data to build a model and
test its accuracy on the remaining 10%. The resulting
average accuracy is likely somewhat of an underestimate
for the true accuracy when the model is trained
on all data and tested on unseen data, but in most cases
this estimate is reliable, particularly if the amount of
labeled data is sufficiently large and if the unseen data
follows the same distribution as the labeled examples.