The probability figure is based on the ratio of bad encounters to good encounters. This value ranges from -1 which indicates a 100% probability that the message will be "ham" and +1 which indicates a 100% probability that the message will be "spam". In the center we have 0 which indicates a 50/50 probability of "spam" or "ham".

The probability is calculates as follows:

Probability = (Bad - Good) / (Bad + Good);