Joint and conditional probabilities

Surveys conducted for the Pew Internet & American Life Project suggest that communication patterns vary across three groups of teenagers: social networkers, content creators, and multichannel teens (teens who use the Internet to send instant messages and participate in social networks and who use their cell phones to send text messages).

In one survey, Internet-using teenagers were asked how often they send text messages to friends. Their responses are summarized in the following table.

	Texts Every Day	Does Not Text Every Day	Total
Social networker	177	316	493
Not a social networker	83	310	393
Total	260	626	886

Consider the following random experiment: An Internet-using teenager is randomly selected and surveyed. The teen is then categorized as a social networker or not a social networker and as someone who sends text messages to friends every day or someone who does not. When answering the following questions, consider the following definitions.

	A simple probabilityis the probability of a single event.
	A joint probabilityis the probability of two or more events occurring simultaneously.
	A conditional probabilityis the probability of one event occurring given that another occurs.

The probability that the randomly selected Internet-using teenager sends text messages to friends every day is . This probability is considered a probability because it is .

The probability that the randomly selected Internet-using teenager is not a social networker who sends text messages to friends every day is . This probability is considered a probability because it is .

The probability that the randomly selected Internet-using teenager sends text messages to friends every day, given that he or she is not a social networker, is . This probability is considered a probability because it is .