Artificial Intelligence Online Tutorials

Consider the following example. This is an artificial example, but is designed to show how we cannot predict how interventions will work given only observational data. That is, determining conditional probabilities will not predict what happens when there are interventions.

There is much debate about whether marijuana is a "gateway drug": taking marijuana leads to people take hard drugs. One may believe this is because taking marijuana is positively correlated with taking hard drugs. Consider the following belief network that provides a model of how marijuana could be correlated with taking hard drugs:

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First turn on monitoring, and monitor each of the variables. Note the probability of each variable.

First, look at the right two variables:

The left most variable forces the Marijuana variable to have a different value.

Before you look at the solution below try to explain why it is acting like this. What is going on here?

Artificial Intelligence online material, ©David Poole and Alan Mackworth, 2010