# Computing a Joint Entry

Assume a new event T (class starts at 9h15).

How to compute an entry in a joint distribution?

* What is P(S ^ \~M ^ L ^ \~R ^ T)?

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## The general case

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So any entry in joint pdf table can be computed. And so **any conditional probability** can be computed.

## Case Study’s

Pathfinder system. (Heckerman 1991, Probabilistic Similarity Networks, MIT Press, Cambridge MA).

* Diagnostic system for lymph-node diseases.
* 60 diseases and 100 symptoms and test-results.
* 14,000 probabilities.
* Expert consulted to make net.
  * 8 hours to determine variables.
  * 35 hours for net topology.
  * 40 hours for probability table values.
* Apparently, the experts found it quite easy to invent the causal links and probabilities.
* Pathfinder is now outperforming the world experts in diagnosis. Being extended to several dozen other medical domains.
