Statistics for Business: Decision Making and Analysis, 3rd Edition

... 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.

$$\begin{array}{ccc}P\left(C\right)& =& P\left(\left\{\text{No}\hspace{0.17em}and\hspace{0.17em}\text{Comcast}\right\}\hspace{0.17em}or\hspace{0.17em}\left\{\text{Yes}\hspace{0.17em}and\hspace{0.17em}\text{Comcast}\right\}\right)\hfill \\ =& 0.009+0.001=0.010\hfill \end{array}$$

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 ...