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# Baseball star David Ortiz-nicknamed "Big Papi"-is known for his ability to deliver hits in high-pressure situations. Here is a two-way table of his hits, walks, and outs in all of his regular-season and post-season plate appearances from 1997 through 2014. Choose a plate appearance at random. Are the events "Hit" and "Post-season" independent? Justify your answer. # Baseball star David Ortiz-nicknamed "Big Papi"-is known for his ability to deliver hits in high-pressure situations. Here is a two-way table of his hits, walks, and outs in all of his regular-season and post-season plate appearances from 1997 through 2014. Choose a plate appearance at random. Are the events "Hit" and "Post-season" independent? Justify your answer.

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Two-way tables asked 2021-03-12
Baseball star David Ortiz-nicknamed "Big Papi"-is known for his ability to deliver hits in high-pressure situations. Here is a two-way table of his hits, walks, and outs in all of his regular-season and post-season plate appearances from 1997 through 2014. Choose a plate appearance at random. Are the events "Hit" and "Post-season" independent? Justify your answer.

## Answers (1) 2021-03-13

DEFINITIONS Two events are independent, if the probability that one event occurs in no way affects the probability of the other event occurring. Definition conditional probability: $$\displaystyle{P}{\left({A}{\mid}{B}\right)}={\frac{{{P}{\left({A}\cap{B}\right)}}}{{{P}{\left({A}\right)}}}}={\frac{{{P}{\left({A}{\quad\text{and}\quad}{B}\right)}}}{{{P}{\left({A}\right)}}}}$$ SOLUTION We note that the table contains information about 8883 at-bats (given in the bottom right corner of the table). Moreover, 2110 of the 8883 at-bats are Hits, because 2110 is mentioned in the row ” Total” and in the column ” Hit” of the table. The probability is the number of favorable outcomes divided by the number of possible outcomes: $$P(Hit) = \frac{\# of\ favorable\ outcomes}{\# of\ possible\ outcomes} = \frac{2110}{8883} \approx 0.2375 = 23.75\%$$
Next, we note that 352 of the 8883 at-bats are post-season, because 352 is mentioned in the row ”Post” and in the column ”Total” of the given table. $$P(Post) = \frac{\# of\ favorable\ outcomes}{\# of\ possible\ outcomes}= \frac{352}{8883}$$
Next, we note that 87 of the 8883 at-bats are hits in post-season, because 87 is mentioned in the row ”Post” and in the column ”Hit” of the given table. $$P(Hit and Post) = \frac{\# of\ favorable\ outcomes}{\# of\ possible\ outcomes} = \frac{87}{8883}$$ Use the definition of conditional probability: $$\displaystyle{P}{\left({H}{i}{t}{\mid}{P}{o}{s}{t}\right)}={\frac{{{P}{\left({H}{i}{t}{\quad\text{and}\quad}{P}{o}{s}{t}\right)}}}{{{P}{\left({P}{o}{s}{t}\right)}}}}$$
$$\displaystyle{\frac{{\frac{{87}}{{8883}}}}{{\frac{{352}}{{8883}}}}}$$
$$\displaystyle{\frac{{{87}}}{{{352}}}}$$
$$\displaystyle\approx{0.2472}$$
$$\displaystyle={24.72}\%$$
If events A and B are independent, then $$P(A|B) = P(A)$$ and $$P(B|A) = P(B)$$
In this case, we note that $$P(Hit|Post) = 0.2472$$ is not the same as $$P(Hit) = 0.2375$$ and thus the two events are not independent.

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