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Poisson distribution

As explained in Natural Language Processing in Action, Second Edition

A Poisson distribution is a probability distribution for counting events in a fixed interval given a specified average rate.

A Poisson distribution models how many events occur in a fixed interval when their average rate is specified. In latent Dirichlet allocation (LDiA), it supplies the document’s generated word count: one parameter sets the average document length. section 4.5.1

Generating documents

A document-generation process first draws its word count from a Poisson distribution, then draws a topic mixture from a Dirichlet distribution and selects words until that count is reached. For a comments corpus, the observed mean length is about 54.21 tokens. section 4.5.1

How Natural Language Processing in Action, Second Edition covers it

How deep it goes
Explains its single average-length parameter and generative role, but does not develop probability formulas or inference details.
Examples
Uses document generation and a comments corpus whose mean tokenized length is approximately 54.21.
What you can do after
Apply it by estimating an average document length and using that value to parameterize document-length generation.
How it connects
Connects document length with Dirichlet distribution, topic mixtures, bag-of-words generation, and term-topic probabilities.

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