Our previous work developed an online learning Bayesian framework (dynamic tree) for data organization and
clustering. To continuously adapt the system during operation, we concurrently seek to perform outlier detection
to prevent them from incorrectly modifying the system. We propose a new Bayesian surprise metric to differentiate
outliers from the training data and thus help to selectively adapt the model parameters. The metric is
calculated based on the difference between the prior and the posterior distributions on the model when a new
sample is introduced. A good training datum would sufficiently but not excessively change the model; consequently,
the difference between the prior and the posterior distributions would be reasonable to the amount of
new information present on the datum. However, an outlier carries an element of surprise that would significantly
change the model. In such a case, the posterior distribution would greatly differ from the prior resulting in a large
value for the surprise metric. We categorize this datum as an outlier and other means (e.g. human operator) will
have to be used to handle such cases. The surprise metric is calculated based on the model distribution, and as
such, it adapts with the model. The surprise factor is dependent on the state of the system. This speeds up the
learning process by considering only the relevant new data. Both the model parameters and even the structure
of the dynamic tree can be updated under this approach.
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