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# Bayesian methods

## Basic probability calculus

We say that the probability of an event occurring is $$P(A)$$ where $$A$$ is the event. Note that $$0 \leq P(A) \leq 1$$ by convention. For example, the probability of a fair coin landing heads up is $$P(H) = 0.5$$, and perhaps the probability of a lightning storm today is $$P(L) = 0.01$$. $$P(\neg A)$$ is always equal to $$1.0 - P(A)$$.

Sometimes we want to describe the probability of some event given that we know (or are supposing) that some other event occurred. For example, the probability of a lightning storm is higher if the sky is cloudy, i.e., $$P(L|C) > P(L|\neg C)$$ (where $$C$$ means the sky is cloudy and $$\neg C$$ means it is not).

If two events $$A$$ and $$B$$ are independent (e.g., knowing that $$A$$ occurred tells you nothing about whether $$B$$ occurred), then $$P(A, B) = P(A)P(B)$$.

If $$A$$ and $$B$$ are not independent, then they are conditional, e.g., knowing that $$A$$ occurred changes the probability that $$B$$ occurred. Now we have,

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

where $$P(A|B)$$ means “probability of $$A$$ given that $$B$$ is known to have occurred.”

Of course, if $$A$$ and $$B$$ are independent, then $$P(A|B)=P(A)$$, so we’re back to the definiton of independent probabilities.

## Bayes’ insight

Notice that $$P(A, B) = P(B, A)$$ (always), simply because the comma doesn’t add any meaning, it’s just a way of writing “$$A$$ and $$B$$ both occurred (and not in some special temporal sequence).”

Thus, the conditional probability of $$P(B,A)$$ is,

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

which means,

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

and rewriting we get,

$$P(B|A) = \frac{P(A|B)P(B)}{P(A)}.$$

So who cares? Well, what we just discovered is that the conditional probability of $$B$$ given $$A$$ (i.e., $$P(B|A)$$), depends entirely on the conditional probability of $$A$$ given $$B$$ and the prior probabilities of $$A$$ and $$B$$ (i.e., $$P(A)$$ means how often does $$A$$ occur with or without $$B$$ occurring also? same question for $$P(B)$$).

If we take $$A$$ and $$B$$ to be events like “$$A$$ = your house alarm is making noise” and “$$B$$ = a burglar is inside your house,” then we have that “the probability that a burglar is inside your house given that you know your house alarm is making noise depends on the chance of your house alarm making noise given a burglar being in your house ($$P(A|B)$$), the prior probability of being robbed at all ($$P(B)$$), and the prior probability of your house alarm making noise from burglars, cats, thunder, etc. ($$P(A)$$).” Note that the prior probability of the alarm making noise ($$P(A)$$) is at least as large as $$P(A|B)P(B)$$, which is the numerator of this fraction. This is because $$P(A)=P(A|B)P(B)+P(A|\neg B)P(\neg B)$$, in other words, the probability of the alarm making noise is the probability of it making noise due to a burglar plus the probability of it making noise due to some other cause. We use “plus” rather than “times” here because we are describing two separate causes for the alarm making noise; these causes are not required to happen simultaneously.

Bam! We have a way of calculating the probability of events we’re not sure about (the chance there is a burglar in my house) given the probabilities of events we do know about. This micro insight has revolutionized computer science, medicine, pyschology, etc. etc., beginning in the 1980s or so. The person who made these and more complex calculations achievable in computer software, Judea Pearl, was awarded the Turing Award in 2012. The importance of Bayes’ micro insight cannot be overstated.

## Naïve Bayesian classification

### Feature vectors

Documents are simply binary word vectors. No tf-idf transformation is done.

### Category vectors

Each category vector is represented as a series of probabilities, one probability per word (each vector dimension represents a word, just like a document feature vector). Each probability means, “the probability of this word being present in a document that is a member of this category.” Thus, the category vector has terms $$C_c = (p_{c1}, p_{c2}, \dots, p_{ck})$$, and

$$p_{ci} = P(w_i|C_c) = \frac{d_{ci}+1}{d_{i}+|C|},$$

where $$d_{ci}$$ is the number of documents in $$C_c$$ that have word $$i$$ (anywhere in the document, any number of occurrences), $$d_i$$ is the number of documents in any category that have word $$i$$, and $$|C|$$ is the number of categories. We add 1 and add $$|C|$$ so that $$P(w_i|C_c)$$ is never equal to 0. (This is called Laplace smoothing.)

### Algorithm

We assume, for simplicity, that the occurrences of words in documents are completely independent (this is what makes the method “naïve”). This is patently false since, for instance, the words “vision” and “image” often both appear in documents about computer vision; so seeing the word “vision” suggests that “image” will also appear in the document.

We further assume that the order the words appear in the document does not matter.

Because we make this independence assumption, we can calculate the probability of a document being a member of some category quite easily:

$$P(\hat{X}|C_c) = \prod_i P(w_i|C_c),$$

where $$P(w_i|C_c) = p_{ci}$$ (from the definition above).

Now, Bayes’ theorem gives us:

$$P(C_c|\hat{X}) = P(\hat{X}|C_c)P(C_c) / P(\hat{X}),$$

with,

$$P(C_c) = \frac{n_c + 1}{n + |C|},$$

where $$n_c$$ is the number of documents in category $$C_c$$ and $$n$$ is the number of documents overall. Again, we use Laplace smoothing; this allows us to avoid probabilities equal to 0.0.

Since we want to find the category $$C_c$$ that makes the quantity maximal, we can ignore $$P(\hat{X})$$ because it does not change depending on which category we are considering.

Thus, we are actually looking for:

$$\arg\max_{C_c} P(C_c|\hat{X}) = \arg\max_{C_c} P(\hat{X}|C_c)P(C_c)$$

We just check all the categories, and choose the single best or top $$N$$.

### A problem with tiny values

With a lot of unique words, we create very small values by multiplying many $$p_{ci}$$ terms. On a computer, the values may become so small that they may “underflow” (run out of bits required to represent the value). To prevent this, we just throw a logarithm around everything:

$$\log P(\hat{X}|C_c)P(C_c) = \log P(\hat{X}|C_c) + \log P(C_c),$$

and furthermore,

$$\log P(\hat{X}|C_c) = \log \prod_i P(w_i|C_c) = \sum_i \log P(w_i|C_c)$$

So our multiplications turn to sums, and we avoid the underflow problem. Rewriting again, we ultimately have this problem:

$$\arg\max_{C_c} (\sum_i \log P(w_i|C_c)) + \log P(C_c)$$

### Evaluation

These graphs show the performance of the naïve Bayes approach on various datasets (compare with results from the document classification notes). The calculations described above are represented as the “binary” algorithm in the graphs. The “tfidf” algorithm, as applied in a naïve Bayes context, uses slightly different calculations that have not been described in these notes.

The book Introduction to Information Retrieval gathered some published results for classification tasks. We can see that naïve Bayes is usually not as good as k-nearest neighbor (which we did learn about) nor support vector machines (which we didn’t learn about).

DatasetNaïve Bayesk-nearest neighborSupport vector machines
earn0.960.970.98
acq0.880.920.94
money-fx0.570.780.75
grain0.790.820.95
crude0.800.860.89
interest0.650.740.78
ship0.850.790.86
wheat0.700.770.92
corn0.650.780.90

I also performed a Spam detection experiment, using the Spambase dataset. With naïve Bayes I was able to achieve ~80% accuracy.

### Benefits of naïve Bayes

• It is very fast. In the table above, while naïve Bayes does not perform as well, it is significantly more efficient than either k-nearest neighbor or support vector machines. The latter, support vector machines, are painfully slow (at least in the training phase).

### Drawbacks of naïve Bayes

• Accuracy is low, as seen in the table above.