The algorithm of probability can be found in the User's Guide for SAS Sentiment Analysis. Here is what the documentation states for the calculation of the probability:
for each positive rule match:
overall_sentiment = overall_sentiment + (weight of the rule * common weighting factor for positive rules)
for each negative rule match:
overall_sentiment = overall_sentiment – (weight of the rule)
overall_sentiment_probability =1/(1+e((0-overall_sentiment)*log(1.5)))