# Anomaly Detection - Class Review

Last updated on：2 years ago

*Anomaly detection refers to the problem of finding patterns in data that do not conform to expected behaviour.* It can be utilized in fraud detection, medical and public health anomaly detection, industrial damage detection, image processing, etc.

## Fraud detection

$x^{(i)}$ = features of user $i$ ‘s activities

Model $p(x)$ from data

Identify unusual users by checking which have $p(x)<\varepsilon$

# Anomaly detection algorithm

- Choose features $x_i$ that you think might be indicative of anomalous examples
- Fit parameters $\mu_j, \sigma_j^2$, Vectorize: $\mu = \frac{1}{m}\sum^m_{i=1} x^{(i)}$
- Given new example $x$, compute $p(x)$, anomaly if $p(x)<\epsilon$

## Developing an evaluating an anomaly detection system

The importance of real-number evaluation, when 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 labelled data, of anomalous and non-anomalous examples.

## Algorithm evaluation

Fit model p(x) on the training set

on a cross validation/test example x, predict y

Possible evaluation metrics

Can also use cross validation $\epsilon$

## Anomaly detection vs supervised learning

If it is a large volume of bad examples, you can shift over to supervised learning.**Anomaly detection**

Fraud detection + manufacturing + monitoring machines in a data centre

**Supervised learning**

Email spam classification + weather prediction + cancer classification

Anomaly detection | Supervised learning |
---|---|

Very small number of positive examples ($y = 1$). (0 - 20 is common) | Large number of positive and negative examples. |

Large number of negative ($y = 0$) examples. | |

Many different “types” of anomalies. 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 likely to be similar to ones in training set. |

## Density estimation

$$p(x) = p(x_1;\mu_1, \sigma_1^2)p(x_2;\mu_2, \sigma_2^2) … p(x_n;\mu_n, \sigma_n^2)$$

$$= \Pi^n_{j=1}p(x_j;\mu_j, \sigma_j^2)$$

$$x_1 ~ N(\mu_1, \sigma_1^2)$$

$$x_2 ~ N(\mu_2, \sigma_2^2)$$

$$x_n ~ N(\mu_n, \sigma_n^2)$$

**Choose what features to use**

$$log(x_j + constant) \to x_j$$

$$\sqrt{x_j} \to x_j$$

# Class exercises

## Within classes

**Q1:** Suppose your anomaly detection algorithm is performing poorly and outputs a large value of $p(x)$ for many normal examples and for many anomalous examples in your cross validation dataset. Which of the following changes to your algorithm is most likely to help?

- [ ] Try using fewer features.

- [x] Try coming up with more features to distinguish between the normal and the anomalous examples.

- [ ] Get a larger training set (of normal examples) with which to fit $p(x)$.

- [ ] Try changing $\varepsilon$.

## Homework

**Q1:** For which of the following problems would anomaly detection be a suitable algorithm?

From a large set of hospital patient records, predict which patients have a particular disease (say, the flu}.

- [x] In a computer chip fabrication plant, identify microchips that might be defective.

- [ ] Given data from credit card transactions, classify each transaction according to type of purchase [for example: food, transportation, clothing}.

- [x] From a large set of primary care patient records, identify individuals who might have unusual health conditions.

- [ ] Given an image of a face, determine whether or not it is the face of a particular famous individual.

*This problem is more suited to traditional supervised learning, as you want both famous and non-famous images in the training set.*

**Q3:** Suppose you are developing an anomaly detection system to catch manufacturing defects in airplane engines You model uses

$$p(x) = \Pi^n_{j=1}p(x_j; \mu_j, \sigma_j^2)$$

You have two features $x_1$ = vibration intensity and $x_2$ = heat generated Both $x_1$ and $x_2$ take on values between 0 and 1 (and are strictly greater than 0), and for most normal engines you expect that:

$x_1 \approx x_2$. One of the suspected anomalies is that a ﬂawed engine may vibrate very intensely even without generating much heat (large $x_1$ small $x_2$) even though the particular values of $x_1$ and $x_2$ may not fall outside their typical ranges of values What additional feature $x_3$, should you create to capture these types of anomalies:

- [x] $x_3 = \frac{x_1}{x_2}$

- [ ] $$x_3 = x_1 + x_2$$

**Q4:** Which of the following are true? Check all that apply.

- [ ] If you are developing an anomaly detection system, there is no way to make use of labelled data to improve your system.

- [x] When choosing features for an anomaly detection system, it is a good idea to look for features that take on unusually large or small values for {mainly the] anomalous examples. (*These are good features, as they will lie outside the learned model, so you will have small values for p(x) with these examples.*)

- [ ] If you have a large labelled training set with many positive examples and many negative examples, the anomaly detection algorithm will likely perform just as well as a supervised learning algorithm such as an SVM.

- [x] If you do not have any labelled data {or if all your data has label y = 0), then is is still possible to learn but it may be harder to evaluate the system or choose a good value of $\varepsilon$. (*Only negative examples are used in training, but it is good to have some labelled data of both types for cross-validation*.)

**Q5:** You have a 1-D dataset ${x^{(1)}, …, x^{(m)}}$ and you want to detect outliers in the dataset. You first plot the dataset and it looks like this:

Suppose you fit the gaussian distribution parameters $\mu_1$ and $\sigma_1^2$ to this dataset. Which of the following values for $\mu_1$ and $\sigma_1^2$ might you get?

- [x] $$\mu_1 = -3, \sigma_1^2 = 4$$

This is correct, as the data are centred around -3 and tail most of the points lie in [-5, -1].

# Reference

[1] Andrew NG, Machine learning

[2] Chandola, V., Banerjee, A. and Kumar, V., 2009. Anomaly detection: A survey. *ACM computing surveys (CSUR)*, *41*(3), pp.1-58.

[3] Chalapathy, R. and Chawla, S., 2019. Deep learning for anomaly detection: A survey. *arXiv preprint arXiv:1901.03407*.

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