Machine learning/Classification algorithms

Classification is a subcategory of supervised learning problems.

k-nearest neighbor

• a simple classification algorithm
• Intuition: Find the majority vote in the training data
• This is a discriminative model, meaning that there is no way to generate the training data points

Algorithm

• Define some distance metric or similarity metric. The simplest case is Euclidean distance.
• Given some input point $x$, find the $k$'th nearest neighbors from the training set.
• Do a majority vote between these nearest neighbor list and classify the input point as the category with highest number of vote.

Probabilistic interpretation

Consider the classification output as a random variable $y$. Define probability of $y$ given input $x$ and training data $D$ is

$$P(y|x,D)={\text{fraction of points }}x_{i}{\text{ in }}k{\text{-th nearest neighbor points to }}x{\text{ such that }}y_{i}=y$$The output of the classification is

$${\hat {y}}=\arg \max _{y}P(y|x,D)$$Read more about probabilistic interpretation here:

• https://www.cc.gatech.edu/~afb/classes/CS7616-Spring2014/slides/CS7616-13a-PKNN.pdf