Does it take longer now than before to be removed from an em

Does it take longer now than before to be removed from an email mailing list? A survey of 100 large online retailers revealed the following:

NEED THREE OR MORE CLICKS TO BE REMOVED

YEAR YES NO

2009 39 61

2008 7 93

If one retailer is selected at random, what is the probability that:

a. You needed 3 or more clicks to be removed from an email list

b. You needed 3 or more clicks to be removed in 2009?

c. You needed 3 or more clicks to be removed from an email list, or were a large online retailer surveyed in 2009?

d. Explain the difference in results in b & c.

Solution

Consider the table:

a) P(3 or more) = 46/200 = 0.23 [answer]

b) P(3 or more|2009) = 39/100 = 0.39 [answer]

c) P(3 or more, OR 2009) = (39+7+61)/200 = 0.535 [answer]

d) In part b, you are only considering the row for year 2009. In part c, you are considering the whole table.