# Classification

## Labelling losses, fitting classifiers etc ## Multi-label

Precision/Recall and f-scores all work for multi-label classification, although they have bad qualities in unbalanced classes.

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## Metric Zoo

One of the less abstruse summaries of these is the scikit-learn classifier loss page, which includes both formulae and verbal descriptions; this is surprisingly hard to find on, e.g. the documentation for deep learning toolkits, in keeping with the field’s general taste for magical black boxes. The Pirates guide to various scores provides an easy introduction.

### Matthews correlation coefficient

Due to Matthews (Matthews 1975) This is the first choice for seamlessly handling multi-label problems, since its behaviour is reasonable for 2 class or multi class, balanced or unbalanced, and it’s computationally cheap. Unless you have a vastly different importance for your classes, this is a good default.

However, it is not differentiable with respect to classification certainties, so you can’t use it as, e.g., a target in neural nets; Therefore you use surrogate measures which are differentiable and use this to track your progress.

#### 2-class case

Take your $$2 \times 2$$. confusion matrix of true positive, false positives etc.

${\text{MCC}}={\frac {TP\times TN-FP\times FN}{{\sqrt {(TP+FP)(TP+FN)(TN+FP)(TN+FN)}}}}$

$|{\text{MCC}}|={\sqrt {{\frac {\chi ^{2}}{n}}}}$

#### Multiclass case

Take your $$K \times K$$ confusion matrix $$C$$, then

${\text{MCC}}={\frac {\sum _{k}\sum _{l}\sum _{m}C_{kk}C_{lm}-C_{kl}C_{mk}}{{\sqrt {\sum _{k}(\sum _{l}C_{kl})(\sum _{k'|k'\neq k}\sum _{l'}C_{k'l'})}}{\sqrt {\sum _{k}(\sum _{l}C_{lk})(\sum _{k'|k'\neq k}\sum _{l'}C_{l'k'})}}}}}$

### ROC/AUC

Receiver Operator Characteristic/Area Under Curve. Supposedly dates back to radar operators in the mid-century. (Matthews 1975) talk about the AUC for radiology; Supposedly (Matthews 1975)introduced it to machine learning, but I haven’t read the article in question. Allows you to trade off importance of false positive/false negatives.

### Cross entropy

I’d better write down form for this, since most ML toolkits are curiously shy about it.

Let $$x$$ be the estimated probability and $$z$$ be the supervised class label. Then the binary cross entropy loss is

$\ell(x,z) = -z\log(x) - (1-z)\log(1-x)$

If $$y=\operatorname{logit}(x)$$ is not a probability but a logit, then the numerically stable version is

$\ell(y,z) = \max\{y,0\} - y + \log(1+\exp(-|x|))$

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## Philosophical connection to semantics

Since semantics is what humans call classifiers.

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