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Outlier Detection (2): Proximity-Based Methods
An object is an outlier if the nearest neighbors of the object are far away, i.e., the proximity of the object is significantly deviates from the proximity of most of the other objects in the same data set
The effectiveness of proximity-based methods highly relies on the proximity measure.
In some applications, proximity or distance measures cannot be obtained easily.
Often have a difficulty in finding a group of outliers which stay close to each other
Two major types of proximity-based outlier detection
Distance-based vs. density-based
Example (right figure): Model the proximity of an object using its 3 nearest neighbors
Objects in region R are substantially different from other objects in the data set.