K nearest neighbor and lazy learning
The nearest neighbour classifier works as follows. Given a new data point whose class label is unknown, we identify the k nearest neighbours of the new data point that exist in the labeled dataset (using some distance function). We then check the class labels of these k nearest neighbours and select the most frequently occurring one (just within the k nearest neighbours, not in the entire dataset). This is the prediction for the class label of the new data point.
For example, consider the following dataset, where income is a numeric feature variable and loan is a binary class label, denoting whether a particular customer with a particular income was able to pay off his/her loan:
Question: Using Euclidean distance, what prediction will 1-nn and 3-nn give for a new data point with income=39.5?
Answer: 1nn = one nearest neighbour. The closest point to 39.5 is the one with income=40 and loan=yes, so predict loan=yes. 3nn = 3 nearest neighbours. The three closest points to 39.5 are those with income=40, 42 and 43. Two of them have loan=yes, one has loan=no, so take the majority vote and predict loan=yes.
Notes about the nearest neighbour classifier: - It is often called “lazy learning”. It does not build any “model”. Instead, all the work (i.e., finding the nearest neighbours) is done at prediction time. - How to choose a good value of k for a particular dataset/application? Try several different values and compute the error rate using cross-validation.