19 3.2 Independent and Mutually Exclusive Events

Independent and mutually exclusive do not mean the same thing.

Independent Events

Two events are independent if the following are true:

• P(A|B) = P(A)
• P(B|A) = P(B)
• P(A AND B) = P(A)P(B)

Two events A and B are independent if the knowledge that one occurred does not affect the chance the other occurs. For example, the outcomes of two roles of a fair die are independent events. The outcome of the first roll does not change the probability for the outcome of the second roll. To show two events are independent, you must show only one of the above conditions.

If two events are NOT independent, then we say that they are dependent.

Sampling may be done with replacement or without replacement.

• With replacement: If each member of a population is replaced after it is picked, then that member has the possibility of being chosen more than once. When sampling is done with replacement, then events are considered to be independent, meaning the result of the first pick will not change the probabilities for the second pick.
• Without replacement: When sampling is done without replacement, each member of a population may be chosen only once. In this case, the probabilities for the second pick are affected by the result of the first pick. The events are considered to be dependent or not independent.

If it is not known whether A and B are independent or dependent, assume they are dependent until you can show otherwise.

You have a fair, well-shuffled deck of 52 cards. It consists of four suits. The suits are clubs, diamonds, hearts and spades. There are 13 cards in each suit consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, J (jack), Q (queen), K (king) of that suit.

1. Sampling with replacement:Suppose you pick three cards with replacement. The first card you pick out of the 52 cards is the
   Q of spades. You put this card back, reshuffle the cards and pick a second card from the 52-card deck. It is the ten of clubs. You put this card back, reshuffle the cards and pick a third card from the 52-card deck. This time, the card is the Q of spades again. Your picks are {Q of spades, ten of clubs, Q of spades}. You have picked the Q of spades twice. You pick each card from the 52-card deck.
2. Sampling without replacement:Suppose you pick three cards without replacement. The first card you pick out of the 52 cards is the
   K of hearts. You put this card aside and pick the second card from the 51 cards remaining in the deck. It is the three of diamonds. You put this card aside and pick the third card from the remaining 50 cards in the deck. The third card is the J of spades. Your picks are {K of hearts, three of diamonds, J of spades}. Because you have picked the cards without replacement, you cannot pick the same card twice.

Example 1

You have a fair, well-shuffled deck of 52 cards. It consists of four suits. The suits are clubs, diamonds, hearts and spades. There are 13 cards in each suit consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, J (jack), Q (queen), K (king) of that suit. Three cards are picked at random.

1. Suppose you know that the picked cards are Q of spades, K of hearts and Q of spades. Can you decide if the sampling was with or without replacement?
   
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   Sampling with replacement

2. Suppose you know that the picked cards are Q of spades, K of hearts, and J of spades. Can you decide if the sampling was with or without replacement?
   
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   No, we cannot tell if the sampling was with or without replacement.

Example 2

You have a fair, well-shuffled deck of 52 cards. It consists of four suits. The suits are clubs, diamonds, hearts, and spades. There are 13 cards in each suit consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, J (jack), Q (queen), and K (king) of that suit. S = spades, H = Hearts, D = Diamonds, C = Clubs.

1. Suppose you pick four cards, but do not put any cards back into the deck. Your cards are QS, 1D, 1C, QD.
2. Suppose you pick four cards and put each card back before you pick the next card. Your cards are KH, 7D, 6D, KH.

Which of 1 or 2 did you sample with replacement and which did you sample without replacement?

This video provides a brief lesson on finding the probability of independent events.

Mutually Exclusive Events

A and B are mutually exclusive events if they cannot occur at the same time. This means that A and B do not share any outcomes and P(A AND B) = 0.

For example, suppose the sample space S = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}.
Let A = {1, 2, 3, 4, 5}, B = {4, 5, 6, 7, 8}, and C = {7, 9}.

1. A AND B = {4, 5}. [latex]\displaystyle{P}{({A} \text{ AND } {B})}=\frac{{2}}{{10}}[/latex]and is not equal to zero. Therefore, A and B are not mutually exclusive.
2. A AND C do not have any numbers in common so P(A AND C) = 0. Therefore, A and C are mutually exclusive.

If it is not known whether A and B are mutually exclusive, assume they are not until you can show otherwise. The following examples illustrate these definitions and terms.

Example 3

Flip two fair coins. (This is an experiment.)

The sample space is {HH, HT, TH, TT} where T = tails and H = heads. The possible outcomes are HH, HT, TH, and TT. The outcomes HT and TH are different. The HT means that the first coin showed heads and the second coin showed tails. The TH means that the first coin showed tails and the second coin showed heads.

• Let A = the event of getting at most one tail. (At most one tail means zero or one tail.) Then A can be written as {HH, HT,TH}. The outcome HH shows zero tails. HT and TH each show one tail.
• Let B = the event of getting all tails. B can be written as {TT}. B is the complement of A, so B = A′.
  P(A) + P(B) = P(A) + P(A′) = 1.
• The probabilities for A = P(event A) = [latex]\frac{{3}}{{4}}[/latex].
• Let C = the event of getting all heads. C = {HH}. Are eevnt C and event B mutually exclusive?
  
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  Since B = {TT}, P(B AND C) = 0.
  B and C are mutually exclusive. (B and C have no members in common because you cannot have all tails and all heads at the same time.)

• Let D = event of getting more than one tail. D = {TT}. Find P(D).
  
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  P(D) = [latex]\frac{{1}}{{4}}[/latex]

• Let E = event of getting a head on the first roll. (This implies you can get either a head or tail on the second roll.) Find P(E).
  
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  E = {HT,HH}.
  P(E) = [latex]\frac{{2}}{{4}}[/latex]

• Find the probability of getting at least one (one or two) tail in two flips.
  
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  Let F = event of getting at least one tail in two flips.
  F = {HT, TH, TT}.
  P(F) = [latex]\frac{{3}}{{4}}[/latex]

Example 4

Flip two fair coins. Find the probabilities of the events.

1. Let F = the event of getting at most one tail (zero or one tail).
2. Let G = the event of getting two faces that are the same.
3. Let H = the event of getting a head on the first flip followed by a head or tail on the second flip.
4. Are F and G mutually exclusive?
5. Let J = the event of getting all tails. Are J and H mutually exclusive?

This video provides two more examples of finding the probability of events that are mutually exclusive.

Example 5

Roll one fair, six-sided die. The sample space is {1, 2, 3, 4, 5, 6}. Let event
A = a face is odd. Then A = {1, 3, 5}. Let event B = a face is even. Then B = {2, 4, 6}.

• Find the complement of A, A′.
  
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  The complement of A, A′, is B because A and B together make up the sample space.
  P(A) +P(B) = P(A) + P(A′) = 1.
  Also, P(A) = [latex]\displaystyle\frac{{3}}{{6}}[/latex].

• Let event C = odd faces larger than two. Let event D = all even faces smaller than five.
  Are C and D mutually exclusive events? Why?
  Show Answer

  C = {3, 5}.D = {2, 4}.
  P(C AND D) = 0 because you cannot have an odd and even face at the same time.
  Therefore, C and D are mutually exclusive events.

• Let event E = all faces less than five.
  Are C and E mutually exclusive events? Why?
  Show Answer

  C = {3, 5} and E = {1, 2, 3, 4}.
  C and E = {3}
  P(C AND E) = [latex]\frac{{1}}{{6}}[/latex].
  No, event C and event E are not mutually exclusive events.

• Find P(C|A).
  
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  This is a conditional probability.
  Recall that the event C is {3, 5} and event A is {1, 3, 5}.
  To find P(C|A), find the probability of C using the sample space A.
  You have reduced the sample space from the original sample space {1, 2, 3, 4, 5, 6} to {1, 3, 5}.
  So, P(C|A) = [latex]\displaystyle\frac{{2}}{{3}}[/latex].

Example 6

Let event G = taking a math class. Let event H = taking a science class. Then, G AND H = taking a math class and a science class. Suppose P(G) = 0.6, P(H) = 0.5, and P(G AND H) = 0.3. Are G and H independent?

• Method 1:

  Show that P(G|H) = P(G).
  P(G|H) = [latex]\frac{\text{P(G and H)}}{\text{P(H)}}[/latex] = [latex]\frac{0.3}{0.5}[/latex] = 0.6
  P(G) = 0.6
  Since P(G|H) = P(G), G and H are independent.

• Method 2:

  Show P(G AND H) = P(G)P(H).
  P(G and H) = 0.3
  P(G) * P(H) = 0.6 * 0.5 = 0.3
  Since P(G AND H) = P(G)P(H), G and H are independent.

• Method 3:

  Show that P(H|G) = P(H).
  P(H|G) = [latex]\frac{\text{P(H and G)}}{P(G)}[/latex] = [latex]\frac{0.3}{0.6}[/latex] = 0.5
  P(H) = 0.5
  Since P(H|G) = P(H), G and H are independent.

Since G and H are independent, knowing that a person is taking a science class does not change the chance that he or she is taking a math class. If the two events had not been independent (that is, they are dependent) then knowing that a person is taking a science class would change the chance he or she is taking math.

Example 7

Let event C = taking an English class. Let event D = taking a speech class.

Suppose P(C) = 0.75, P(D) = 0.3, P(C|D) = 0.75 and P(C AND D) = 0.225.

Justify your answers to the following questions numerically.

1. Are C and D independent?
   
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   Yes, because P(C|D) = P(C).

2. Are C and D mutually exclusive?
   
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   No, because P(C AND D) is not equal to zero.

3. What is P(D|C)?
   
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   [latex]\displaystyle{P}{({D}{mid}{C})}=\frac{{{P}{({C} \text{ AND }{D})}}}{{{P}{({C})}}}=\frac{{0.225}}{{0.75}}={0.3}[/latex]

Example 8

In a box there are three red cards and five blue cards. The red cards are marked with the numbers 1, 2, and 3, and the blue cards are marked with the numbers 1, 2, 3, 4, and 5. The cards are well-shuffled. You reach into the box (you cannot see into it) and draw one card.

Let R = red card is drawn, B = blue card is drawn, E = even-numbered card is drawn.

The sample space S = R1, R2, R3, B1, B2, B3, B4, B5. S has eight outcomes.

• Find P(R).
  
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  P(R) = [latex]\frac{{3}}{{8}}[/latex]. 

• Find P(R and B).
  
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  P(R AND B) = 0. (You cannot draw one card that is both red and blue.)

• Find P(E).
  
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  P(E) = [latex]\frac{{3}}{{8}}[/latex]. (There are three even-numbered cards, R2, B2, and B4.)

• Find P(E|B) .
  
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  P(E|B) = [latex]\frac{\text{P(E and B)}}{P(B)}[/latex] = [latex]\frac{{2}}{{5}}[/latex]
  (There are five blue cards: B1, B2, B3, B4, and B5.
  Out of the blue cards, there are two even cards; B2 and B4.)

• Find P(B|E).
  
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  P(B|E) = [latex]\frac{\text{P(E and B)}}{P(E)}[/latex] = [latex]\frac{{2}}{{3}}[/latex].
  There are three even-numbered cards: R2, B2, and B4.
  Out of the even-numbered cards, to are blue; B2 and B4.

• Are events R and  mutually exclusive?
  
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  The events R and B are mutually exclusive because P(R AND B) = 0.

• Let G = card with a number greater than 3. Find P(G).
  
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  G = {B4, B5}. P(G) = [latex]\frac{{2}}{{8}}[/latex], P(G) = P(G|H), which means that G and H are independent.

Example 9

In a particular college class, 60% of the students are female. Fifty percent of all students in the class have long hair. Forty-five percent of the students are female and have long hair. Of the female students, 75% have long hair. Let F be the event that a student is female. Let L be the event that a student has long hair. One student is picked randomly. Are the events of being female and having long hair independent?

• The following probabilities are given in this example:
• P(F) = 0.60; P(L) = 0.50
• P(F AND L) = 0.45
• P(L|F) = 0.75

Example 10

1. Toss one fair coin (the coin has two sides, H and T). The outcomes are ________. Count the outcomes. There are ____ outcomes.
   
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   H and T; 2

2. Toss one fair, six-sided die (the die has 1, 2, 3, 4, 5 or 6 dots on a side). The outcomes are ________________. Count the outcomes. There are ___ outcomes.
   
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   1, 2, 3, 4, 5, 6 ; 6

3. Multiply the two numbers of outcomes. The answer is _______.
   
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   2(6) = 12

4. If you flip one fair coin and follow it with the toss of one fair, six-sided die, the answer in three is the number of outcomes (size of the sample space). What are the outcomes? (Hint: Two of the outcomes are H1 and T6.)
   
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   T1, T2, T3, T4, T5, T6, H1, H2, H3, H4, H5, H6

5. Event A = heads (H) on the coin followed by an even number (2, 4, 6) on the die.
   A = {_________________}. Find P(A).
   Show Answer

   A = {H2, H4, H6}; P(A) = [latex]\frac{{3}}{{12}}[/latex]

6. Event B = heads on the coin followed by a three on the die. B = {________}. Find P(B).
   
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   B = {H3}; P(B) = [latex]\frac{{1}}{{12}}[/latex]

7. Are A and B mutually exclusive?
   (Hint: What is P(A AND B)? If P(A AND B) = 0, then A and B are mutually exclusive.)
   Show Answer

   Yes, because P(A AND B) = 0

8. Are A and B independent?
   (Hint: Is P(A AND B) = P(A)P(B)? If P(A AND B) = P(A)P(B), then A and B are independent. If not, then they are dependent).
   Show Answer

   P(A AND B) = 0.
   P(A) * P(B) = [latex]\frac{3}{12}[/latex] * [latex]\frac{1}{12}[/latex] = [latex]\frac{3}{144}[/latex] .
   P(A AND B) [latex]\ne[/latex] P(A)P(B), so A and B are dependent.

References

Lopez, Shane, Preety Sidhu. “U.S. Teachers Love Their Lives, but Struggle in the Workplace.” Gallup Wellbeing, 2013. http://www.gallup.com/poll/161516/teachers-love-lives-struggle-workplace.aspx (accessed May 2, 2013).

Data from Gallup. Available online at www.gallup.com/ (accessed May 2, 2013).

Concept Review

Two events A and B are independent if the knowledge that one occurred does not affect the chance the other occurs. If two events are not independent, then we say that they are dependent.

In sampling with replacement, each member of a population is replaced after it is picked, so that member has the possibility of being chosen more than once, and the events are considered to be independent. In sampling without replacement, each member of a population may be chosen only once, and the events are considered not to be independent. When events do not share outcomes, they are mutually exclusive of each other.

Formula Review

If A and B are independent, P(A AND B) = P(A)P(B), P(A|B) = P(A) and P(B|A) = P(B).

If A and B are mutually exclusive, P(A OR B) = P(A) + P(B) and P(A AND B) = 0.