nltk.WittenBellProbDist¶

class nltk.WittenBellProbDist(freqdist, bins=None)[source]¶

The Witten-Bell estimate of a probability distribution. This distribution allocates uniform probability mass to as yet unseen events by using the number of events that have only been seen once. The probability mass reserved for unseen events is equal to T / (N + T) where T is the number of observed event types and N is the total number of observed events. This equates to the maximum likelihood estimate of a new type event occurring. The remaining probability mass is discounted such that all probability estimates sum to one, yielding:

• p = T / Z (N + T), if count = 0
• p = c / (N + T), otherwise