Festival’s POS tagger

[Tushar S]

What does this part of festival’s POS tagger do? It’s not setting the probabilities since all the numbers are negative?

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[Simon]

Those are log probabilities. It is good practice to store probabilities as their logarithm because absolute probabilities can be very small numbers which are hard to store with sufficient precision.

[Tushar S]

SO when it says (nnp -2.9144) that means the probability of a proper noun occuring is 10^-2.9144? For HMMs we need transition probabilities of observed states and emission states but the probabilities given aren’t transition probabilities in any sense.

[Simon]

Yes, that’s how you convert a log probability back to a probability (noting that logs are not necessarily base 10, although in Festival I believe they are).

“transition probabilities of observed states and emission states” is a little muddled – I suggest waiting for HMMs to be covered in the Speech Processing course and then see if that helps you understand better. States emit observations according to some emission probability distribution, and there are also probabilities on the transitions between states.

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