On the independence of digits in connected digit strings

One of the frequently used assumptions in Speaker Veriication is that two speech segments (phonemes, subwords, words) are considered to be independent. And therefore, the log-likelihood of a test utterance is just the sum of the log-likelihoods of the speech segments in that utterance. This paper reports about cases in which this observation-independence assumption seems to be violated, namely for those test utterances which call a certain speech model more than once. For example, a pin code which contains a non-unique digit set performs worse in veriication than a pin code which consists of four diierent digits. Results illustrate that violating the independence assumption too much might result in increasing EERs while more information (in form of digits) is added to the test utterance.