... 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.


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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