Concept explainers
Consider a directory of classified advertisements that consists of m pages, where m is very large. Suppose that the number of advertisements per page varies and that your only method of finding out how many advertisements there are on a specified page is to count them. In addition, suppose that there are too many pages for it to be feasible to make a complete count of the total number of advertisements and that your objective is to choose a directory advertisement in such a way that each of them has an equal chance of being selected.
a. If you randomly choose a page and then randomly choose an advertisement from that page, would that satisfy your objective? Why or why not?
Let
Step 1. Choose a page at random. Suppose it is page X. Determine n(X) by counting the number of advertisements on page X.
Step 2. “Accept” page X with
Step 3. Randomly choose one of the advertisements on page X.
Call each pass of the algorithm through step 1 an iteration. For instance, if the first randomly chosen page is rejected and the second accepted, then we would have needed 2 iterations of the algorithm to obtain an advertisement.
b. What is the probability that a single iteration of the algorithm results in the acceptance of an advertisement on page i?
c. What is the probability that a single iteration of the algorithm results in the acceptance of an advertisement?
d. What is the probability that the algorithm goes through k iterations, accepting the jth advertisement on page i on the final iteration?
e. What is the probability that the jth advertisement on page i is the advertisement obtained from the algorithm?
f. What is the expected number of iterations taken by the algorithm?
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FIRST COURSE IN PROBABILITY (LOOSELEAF)
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