... Comcast Google Nextag
Purchase No 0.009 0.934 0.022 0.965
Yes 0.001 0.033 0.001 0.035
Total 0.010 0.967 0.023 1

The marginal probability of a visit from Comcast is the sum of the probabilities in this column.

P(C)=P({NoandComcast}or{YesandComcast})=0.009+0.001=0.010

These probabilities also add because the cells define disjoint events.

Conditional Probability

Table 8.3 shows the joint and marginal probabilities, but these do not directly answer an important question: Which host will deliver the best visitors, those who are more likely to make purchases? The answer to this question requires conditional probabilities.

When working with a table of probabilities, we obtain a conditional probability when we restrict the sample space ...

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