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Notes Module 20.2

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20.2: Independent Events

Definition: Two events, A and B, are independent if the fact that A occurs does not affect the probability of B occurring.

To find the probability of two independent events that occur in sequence, find the probability of each event occurring separately, and then multiply the probabilities. This multiplication

rule is defined symbolically below. Note that multiplication is represented by AND.

Multiplication Rule 1: When two events, A and B, are independent, the probability of both occurring is: P(A and B) = P(A) · P(B)

Experiment 1: A dresser drawer contains one pair of socks with each of the following colors: blue, brown, red, white and black. Each pair is folded together in a matching set. You reach into

the sock drawer and choose a pair of socks without looking. You replace this pair and then choose another pair of socks. What is the probability that you will choose the red pair of socks both times? There are a couple of things to note about this experiment. Choosing a pairs of socks from the drawer, replacing it, and then choosing a pair again from the same drawer is acompound event. Since the first pair was replaced, choosing a red pair on the first try has no effect on the probability of choosing a red pair on the second try. Therefore, these events are independent.

Probabilities:

P(red) = 1

5

P(red and red) = P(red) · P(red)

= 1 · 1

5 5

= 1

25

Some other examples of independent events are:

Landing on heads after tossing a coin AND rolling a 5 on a single 6-sided die.

Choosing a marble from a jar AND landing on heads after tossing a coin.

Choosing a 3 from a deck of cards, replacing it, AND then choosing an ace as the second card.

Rolling a 4 on a single 6-sided die, AND then rolling a 1 on a second roll of the die.

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Definition: Two events, A and B, are independent if the fact that A occurs does not affect the probability of B occurring.

To find the probability of two independent events that occur in sequence, find the probability of each event occurring separately, and then multiply the probabilities. This multiplication

Multiplication Rule 1: When two events, A and B, are independent, the probability of both occurring is: P(A and B) = P(A) · P(B)

Experiment 1: A dresser drawer contains one pair of socks with each of the following colors: blue, brown, red, white and black. Each pair is folded together in a matching set. You reach into

the sock drawer and choose a pair of socks without looking. You replace this pair and then choose another pair of socks. What is the probability that you will choose the red pair of socks both times? There are a couple of things to note about this experiment. Choosing a pairs of socks from the drawer, replacing it, and then choosing a pair again from the same drawer is a compound event. Since the first pair was replaced, choosing a red pair on the first try has no effect on the probability of choosing a red pair on the second try. Therefore, these events are independent.

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Experiment 3: A card is chosen at random from a deck of 52 cards. It is then replaced and a second card is chosen. What is the probability of choosing a jack and then an eight? Probabilities:

P(jack) = 4

52

P(8) = 4

Experiment 2: A coin is tossed and a single 6-sided die is rolled. Find the probability of landing on the head side of the coin and rolling a 3 on the die.

Probabilities:

P(head) = 1

2

P(3) = 1

6

P(head and 3) = P(head) · P(3)

= 1 · 1

2 6

= 1

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52

P(jack and 8) = P(jack) · P(8)

= 4 · 4

52 52

= 16 2704

= 1 169

Experiment 4: A card is chosen at random from a deck of 52 cards. It is then replaced and a second card is chosen. It is then replaced and a third card is chosen.

What is the probability of choosing a jack, then an eight, and then an ace? Probabilities:

P(jack) = 4 52

P(8) = 4

52

P(ace) = 4

52

P(jack,8, and ace) = P(jack) · P(8) · P(ace)

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Testing to see whether events are independent of each other or dependent

If P(A B) = P(A), then events are independent.

The equal results indicate that the probability of event A is unchanged when event B occurs first. Therefore, the events are independent.

If P(A B) = P(A) P(B), then events are independent.

It is also written P(A and B) = P(A) P(B). The P(A and B) refers to the overlap when looking at 2 way table where one sees events being double counted.

Example 1:

Is the event that a flight is on time independent of the event that a flight is domestic?

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Conclusion: The equal results indicate that the two events are independent of one another. Therefore, the events are independent.

Example 2:

Is the event that an apartment has a single occupant independent of the event that an apartment has 1 bedroom?

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