NBest

NBest is a very useful feature for handling similar sounding words. Normally a speech rec engine finds the grammar rule that is the best match for the user's speech utterance. Large grammars can suffer from substitution errors where the wrong rule is matched: caller says "Boston" but the engine selects "Austin". NBest helps the application sort out this type of ambiguity.

When enabled, NBest is a request to the speech rec engine to return the top N matches, sorted in order of decreasing confidence level. N is usually a small number, such as 4. Remember that the NBest value is a maximum; fewer results may be returned.

In SpeakRight, NBest is enabled using the QuestionFlow method enableNBest.

flow.enableNBest(4); //up to 4 results

When the SRResults come back for that question, you can check for NBest results. The SRResults method hasNBest which indicates that more than one result was returned.

NBest Pruning
The simplest thing an application can do, is check the NBest results in validateInput, and use additional application logic to select the most likely result. This is called NBest pruning. For example, if the user is asked for her account number, each result can be checked against the database. If only one result is a valid account number, the application could assume that's what the caller said.

String probableAccountNumber = "";
int matches = 0;
for (int i = 0; i <>
String value = results.getNBestValue(i);
if (CheckAccountNumber(value)) {   //check against the database
probableAccountNumber = value;
matches++;
}
}
if (matches == 1) {
results.replaceInput(probableAccountNumber); //let's use it!
}

NBest Confirmation
A more common use for NBest is to do confirmation. When NBest results are returned, the application confirms each NBest result, stopping as soon as the user says "yes".

C: What city?
H: Boston
C: (returns 'Austin' and 'Boston' as NBest results) Do you want Austin?
H: No
C: Do you want Boston?
H: Yes
C: Great. Flying to Boston on what date?
...

The application may still want to prune the NBest results, re-ordering the results according to the most likely answers. This way the first confirmation question is more likely to be the correct one. This is an important part of NBest -- using additional context information and application logic to improve on the speech rec engine's results.

Pass an NBestConfirmerFlow and the question flow object to a ConfirmationWrapper object that will manage the confirmation process. It will ask the user to confirm values until the caller accepts one (by saying "yes" or whatever your confirmation grammar uses for acceptance). If the caller says "no" to all NBest values, then the question is asked again, and the process repeats. You can override NBestConfirmerFlow to adjust this behaviour.

Note that NBest confirmation is an extension of basic confirmation. A YesNoConfirmerFlow confirms a single result, while a NBestConfirmerFlow confirms multiple results.

ConfirmationWrapper cw = new ConfirmationWrapper(new AskCity(),
new NBestConfirmer("yesno.grxml"));

Skip Lists
A skip list is a list of words that the application will not confirm because the caller has already rejected them. This is an optional feature of NBestConfirmerFlow. Enable it with the enableSkipList method. If the caller says "no" to all NBest values, then the question is asked again. Before beginning confirmation, NBestConfirmerFlow will remove from the new NBest results any values that were rejected during the previous round of confirmation questions. If this results in only a single NBest result, then there is no need for confirmation.

C: What city?
H: Crosston
C: (returns 'Austin' and 'Boston' as NBest results) Do you want Austin?
H: No
C: Do you want Boston?
H: No
C: (asking the question again) Let's try again. What city?
H: Crosston
C: (returns 'Austin' and 'Crosston' and 'Aulston' as NBest results. Austin is removed.)Do you want Crosston?
H: yes
C: Got it. Flying to Crosston on what date?
...

If you don't use a skip list, the application can infuriatingly confirm the same wrong result again and again.

NBest With SROs
SpeakRight Reusable Objects (SROs) are pre-built flow objects for gathering common data such as numbers, dates, etc.

To enable NBest for an SRO, use its enableNBest method. This will use an SROConfirmNBest confirmer object. If you need to use a custom confirmer, call enableNBest followed by setConfirmer to pass in your custom confirmer.

SRONumber flow = new SRONumber("tickets", 1, 10);
flow.enableNBest(4); //up to 4 results