## Support Vector Machine Concept: VC Dimension

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Given a set of n samples \$x_1, x_2, …, x_n\$, we want to label them as either -1 or 1. In total, there are \$2^n\$ possible label combinations.

A class of learning machines H can be used to label the samples. If for each label combination, we can always find a learning machine \$h in H\$ that labels it correctly, we then say that H shatters n points.

VC (Vapnik-Chervonekis) dimension is then defined as the maximum number of points that can be shattered by H, which measures the capacity of the hypothesis class H.

Note that VC(H) = 4 does not mean H can shatter any 4 points in the hyperplane, as long as there’s 4 points can be shattered by H, it’s good enough. And it also infers that for any > 4 (say 5) points in the hyperplane, it cannot be shattered by H.

Now consider a simple example to find the VC dimension of a class of hypothesis H. H is defined as straight line in a two dimensional hyperplane.

We can find the following 3 points a, b and c where all \$2^3\$ possible label combinations can be classified correctly by H.

As mentioned before, it’s possible to find 3 points that H cannot shatter. For example, we just place a, b and c in the same line. There will be combinations that a straight line cannot label them correctly. But this doesn’t affect the VC dimension as it’s defined as the maximum number of points can be shattered.

One can try separate 4 points using a straight line, but no matter where we place those 4 points, there will be combinations that cannot be separated.

## Naive Bayes

#### Bayes’ Theorem

Let’s start from Bayes’ theorem, also referred as Bayes’ law or Bayes’ rule.

P(A|B) = P(B, A) / P(B)

= P(B|A) * P(A) / P(B)

= P(B|A) * P(A) / (P(B|A) * P(A) + P(B|^A) * P(^A))

P(A): prior probability. It’s the probability event A happens.

P(^A): the probability that event A not happen.

P(B): evidence, or background. The probability of event B happens.

P(B|A), P(B|^A): conditional probability, or likelihood. The probability of event B happens given A happened or not happened respectively.

P(A|B): posterior probability. The probability of A happens taking into account B for and against A.

#### Naive Bayes

When used for classification, Bayes’ Theorem can be expressed as below,

P(C|F1, F2, … Fn) = P(C, F1, F2, …, Fn) / P(F1, F2, … , Fn)

= P(F1, F2, … Fn|C) * P(C) / P(F1, F2, …, Fn)

C is some class/label we can classify a sample into, and F1, F2, … Fn represents features of the sample data.

P(F1, F2, …, Fn) doesn’t depend on C and are normally given or can be calculated based on probability of each feature. It’s effectively a constant and can be ignored for classification purpose.

The numerator can be expressed as following,

P(C, F1, F2 … , Fn)

= P(C) * P(F1, F2, … , Fn|C)

= P(C) * P(F1 | C) * P(F2, F3, … Fn | F1, C)

= P(C) * P(F1 | C) * P(F2 | F1, C) * P(F3, … Fn | F1, F2, C)

= P(C) * P(F1 | C) * P(F2 | F1, C) * P(F3 | F1, F2, C) * …  * P(Fn | F1, F2, …, Fn-1, C)

In Naive Bayes, all features are assumed to be independent. Thus Fi is independent from every other feature Fj where j != i. Therefore we have

P(F2 | F1, C) = P(F2 | C)

P(F3 | F1, F2, C) = P(F3 | C)

P(Fn | F1, F2, … Fn-1, C) = P(Fn | C)

Thus,

P(C, F1, F2 … , Fn) = P(C) * P(F1 | C) * P(F2 | C) * P(F3 | C), …, P(Fn | C)

For example, two authors A and B like to use words “love”, “life” and “money”. The probability of these words appears in A’s article is 0.1, 0.1 and 0.8, and in B’s as 0.5, 0.3 and 0.2. Now we have the phrase “love life”, which one of the author is more likely to have written that?

Without any information, there’s 50% percent probability for either A or B. Assuming the words are independent features, we can use Naive Bayes.

P(A | love, life) = P(A) * P(love | A) * P(life | A) / P(love, life) = 0.5 * 0.1 * 0.1 / P(love, life)

P(B | love, life) = P(B) * P(love | B) * P(life | B) / P(love, life) = 0.5 * 0.5 * 0.3 / P(love, life)

Clearly, it’s more likely that the phrase “love life” is written by author B. Note that P(love, life) is independent from the authors and just a scaling factor.

References:

1. Bayes’ theorem: http://en.wikipedia.org/wiki/Bayes%27_theorem
2. Naive Bayes classifier: http://en.wikipedia.org/wiki/Naive_Bayes_classifier
3. Udacity, Intro to Machine Learning. Lesson 1. Naive Bayes.

## Performance Metrics for Binary Classification

Binary classification classifies samples as either 0 (negative) or 1(positive). Depending on which class/label the sample data belongs to and its predication result, each sample fits in a cell of the following table.

 predicted | actual positive negative positive true positive (TP) false positive (FP) negative false negative (FN) true negative (TN)

We can then calculates different metrics, which measures the classification results from different angles.

True Positive Rate (Sensitivity, hit rate, recall): Out of all the positive samples, what fraction is actually detected as positive.

TPR = TP / (TP + FN)

True Negative Rate (Specificity): Out of all the negative samples, what fraction is actually detected as negative.

TNR = TN / (TN + FP)

Positive Predictive Value (Precision): Out of all samples predicted as positive, what fraction is actually positive.

PPV=TP / (TP + FP)

Negative Predictive Value: Out of all samples predicted as negative, what fraction is actually negative.

NPV = TN / (TN + FN)

False Positive Rate (Fall out): Out of all negative samples, what fraction is detected as positive by mistake.

FPR = FP / (FP + TN) = 1 – TNR

False Discovery Rate: Out of all samples predicted as positive, what fraction is actually negative.

FDR = FP / (FP + TP) = 1 – PPV

Accuracy: Out of all samples, what fraction is predicted correctly. That is, positive samples are predicted as positive, negative samples are predicted as negative.

Accuracy = (TP + TN) / (P + N)

The table below gives a nice overall view of the metrics mentioned above,

 predicted | actual positive negative positive true positive (TP) false positive (FP) PPV = TP / (TP + FP) negative false negative (FN) true negative (TN) NPV = TN / (FN + TN) TPR = TP / (TP + FN) TNR = TN / (FP  + TN) Accuracy = (TP + TN) / (P + N)

F1 score: the harmonic mean of precision and recall.

F1 =2 * TPR*PPV / (TPR + PPV) = 2TP / (2TP + FP + FN)

Matthews correlation coefficient: It takes account true and false positives and negatives, and is regarded as balanced measure. It can be used for cases where the number of samples at different classes vary drastically.

MCC = (TP * TN – FP * FN) / √(TP + FP)(TP + FN)(TN + FP)(TN + FN)

MCC returns a value between -1 to 1, where -1 indicates total disagreement between prediction and actual facts, 0 means no better than random guess, and 1 indicates perfect prediction.

References:

1. Matthews correlation coefficient: http://en.wikipedia.org/wiki/Matthews_correlation_coefficient
2. Sensitivity and specificity: http://en.wikipedia.org/wiki/Sensitivity_and_specificity