Please take a look at this link: http://library.bayesia.com/display/BlabC/Discrete+Probability+Distributions
This function may be used when there are several possible causes for an event, any of which can cause the event by itself, but only with a certain probability.
Also, if desired, it allows for the event to occur spontaneously, without any of the known causes being true.
It takes its simplest form when all the parent nodes and the child node are boolean (e.g. true/false).
It is similar to a regular “or” function in which the child takes on state true if any of its parents are true, otherwise it takes on false.
However, there is a probabilistic component (hence the term “noisy”), in that even if a parent is true, the child is not necessarily true.
For each parent there is one number called the causal strength, which gives the probability that the child is true when that parent is true, apart from any other interactions.
It is usually denoted pi for the parent. As well, there is a probability that the child is true even when all the parents are false, called the leak probability (this can simply be made zero if there is no such possibility).
I hope this helps