One important concept of the classification tree is similar to what we saw in the regression model from Part 1: the concept of using a "training set" to produce the model. This takes a data set with known output values and uses this data set to build our model. Then, whenever we have a new data point, with an unknown output value, we put it through the model and produce our expected output. This is all the same as we saw in the regression model. However, this type of model takes it one step further, and it is common practice to take an entire training set and divide it into two parts: take about 60-80 percent of the data and put it into our training set, which we will use to create the model; then take the remaining data and put it into a test set, which we'll use immediately after creating the model to test the accuracy of our model.