15. Anomaly detection

Anomaly detection

Problem motivation

Gaussian distribution

Gaussian distribution: Say $x\in R$ . If $x$ is a distributed Gassian with mean $\mu$ , variance $\sigma^2$

$x\sim N(\mu,\sigma^2)$

$P(x;\mu,\sigma^2) = \frac{1}{\sqrt{2\pi}\sigma}\exp^{(-\frac{(x-\mu)^2}{2\sigma^2})}$

Parameter estimation:

$\mu = \frac{1}{m}\sum\limits_{i=1}^mx^{(i)}$
$\sigma^2 = \frac{1}{m}\sum\limits_{i=1}^m(x^{(i)}-\mu)^2$

$m = m-1$ , whether use $m$ or $m-1$ make very little difference.

Algorithm

Density estimation

$\begin{aligned} P(x) & = P(x_1;\mu_1,\sigma_1^2)P(x_2;\mu_2,\sigma_2^2)...P(x_n;\mu_n,\sigma_n^2) \\ & =\prod_{j=1}^nP(x_j;\mu_j,\sigma_j^2) \end{aligned}$

Anomaly detection algorithm

Choose features $x_i$ that you think might be indicative of anomalous examples.
Fit parameters $\mu_1,...,\mu_n,\sigma_1^2,...,\sigma_n^2$
Given new example $x$ , compute $p(x)$ :
$\begin{aligned} P(x) & = P(x_1;\mu_1,\sigma_1^2)P(x_2;\mu_2,\sigma_2^2)...P(x_n;\mu_n,\sigma_n^2) \\ & =\prod_{j=1}^nP(x_j;\mu_j,\sigma_j^2) \end{aligned}$
Anomaly if $p\le \epsilon$

Developing and evaluating an anomaly detection system

Whem developing a learning algorithm (choosing features, etc.), making decisions is much easier if we have a way of evaluating our learning algorithm.

Assume we have some labeled data, of anomalous and non-anomalous examples.

Training set (normal examples)
cross validiation set (labeled examples)
test set (labeled examples)

Can also use cross validation set to choose parameter $\epsilon$

Anomaly detection vs. supervised learning

Anomaly detection	Supervised learning
Very small number of positive examples; Large number of negative examples	Large number of positive examples and negative examples
Hard for any algorithm to learn from positive examples what the anomalies look like; future anomalies may look nothing like any of the anomalous examples we've seen so far.	Enough positive examples for algorithm to get a sense of what positive examples are like, future positive examples likely to be similar to ones in training set.

Choosing what features to use

Non-gaussian features: make your data more like Gaussian.

Error analysis for anomaly detection

Most common problem: $p(x)$ is comparable (say, both large) for normal and anomalous examples.
Create some new features.
Choose featrues that might take on unusually large or small values in the event of an anomaly.

Multivariate Gaussian distribution

$x\in R^n$ . Don't model $p(x_1),p(x_2),...,$ etc. separately.
Model $p(x)$ all in one go.
Parameters: $\mu\in R^n$ , $\Sigma\in R^{n\times n}$

$p(x;\mu,\sigma) = \frac{1}{(2\pi)^{n/2}\det(\Sigma)^{1/2}}\exp(-\frac{1}{2}(x-\mu)^T\Sigma^{-1}(x-\mu))$

there are some pics that show the multivariate gaussian look like in the video.

Anomaly detection using the multivariate Gaussian distribution

$p(x;\mu,\sigma) = \frac{1}{(2\pi)^{n/2}\det(\Sigma)^{1/2}}\exp(-\frac{1}{2}(x-\mu)^T\Sigma^{-1}(x-\mu))$

$\begin{aligned} \mu & = \frac{1}{m}\sum_{i=1}^mx^{(i)} \\ \Sigma & = \frac{1}{m}\sum_{i=1}^m(x^{(i)l}-\mu)(x^{(i)}-\mu)^T \end{aligned}$

Original model vs. Multivariate Gaussian

original model:

manually create features to capture anomalies where $x_1,x_2$ take unusual combinations of values.
computationally cheaper
ok even if $m$ is small

multivariate Gaussian:

automatically captures correlations between features
computationally more expensive
must have $m\ge n$ , or else $\Sigma$ is non-invertible

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