# Probably About Probability p <.05. Probability. What Is Probability? Probability of Events. Greg C Elvers

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1 Probably About p <.05 Greg C Elvers 1 Inferential statistics allow us to decide if one condition of an experiment produces different results than another condition Inferential statistics are based on the concepts of probability Thus, probability is an essential aspect of statistics 2 What Is? Probabilities often deal with events An event is something that happens E.g. Rolling a 3 on a fair die is an event The probability of an event is given by the ratio of how often that event occurs and how often all events occur 3 of Events When you role a fair, 6 sided die, each of the six faces has an equal chance of coming up Thus, the probability of any single face appearing is given by 1 (how often that event occurs) divided by 6 (the total number of events) Event p(event) 1 1/6 2 1/6 3 1/6 1/6 5 1/6 6 1/6

2 of Events What is the probability of not rolling a 1? There are five events that are not 1 There is a total of six events The probability of not rolling a 1 is 5 / 6 p(1) = 5 / 6 Event p(event) 1 1/6 2 1/6 3 1/6 1/6 5 1/6 6 1/6 5 The Addition Rule The addition rule is used to determine the probability of two or more events occurring E.g., what is the probability that an odd number will appear on the die? For mutually exclusive events, the addition rule is: p(a or B) = p(a) + p(b) Two events are mutually exclusive when both cannot occur at the same time 6 The Addition Rule An odd event occurs whenever the die comes up 1, 3, or 5 These events are mutually exclusive E.g., if it comes up 3, it cannot also be 1 or 5 p(1 or 3 or 5) = p(1) + p(3) + p(5) = 1/6 + 1/6 + 1/6 =.5 Event p(event) 1 1/6 2 1/6 3 1/6 1/6 5 1/6 6 1/6 7 The Addition Rule When events are not mutually exclusive, a different addition rule must be used When events are not mutually exclusive, one or more of the events can occur at the same time Are the events liking cats and liking stats mutually exclusive? Like Stats Dislike Stats Like Cats Dislike Cats

3 The Addition Rule The two events, liking stats and liking cats are not mutually exclusive It is possible for a person to like statistics and either like or dislike cats Like Stats Dislike Stats Like Cats Dislike Cats The Addition Rule When events are not mutually exclusive, the addition rule is given by: p(a or B) = p(a) + p(b) - p(a and B) p(a and B) is the probability that both event A and event B occur simultaneously This formula can always be used as the addition rule because p(a and B) equals zero when the events are mutually exclusive 10 The Addition Rule Why do we subtract p(a and B)? When the events are not mutually exclusive, some events that are A, are also B Those events are counted twice in p(a) + p(b) p(a and B) removes the second counting of the events that are both A and B p(a) p(a and B) p(b) 11 The Multiplication Rule To determine the probability of two (or more) independent events occurring simultaneously, one uses the multiplication rule p(a and B) = p(a) X p(b) Note: This formula can be used to solve the previous addition rule, but only if the events are independent 12

4 Independent Events Two events are said to be independent if the occurrence of one event in no way influences the occurrence of the other event That is, knowing something about whether one event has occurred tells you nothing about whether the other event has occurred E.g., flipping a coin twice E.g., being struck by lightning and having green eyes 13 The Multiplication Rule What is the probability of rolling two dice and having both show 6? p(6 and 6) = p(6) X p(6) = 1/6 X 1/6 = 1/36 Event p(event) 1 1/6 2 1/6 3 1/6 1/6 5 1/6 6 1/6 1 Some Problems What is the probability of selecting a King in a single draw from a standard deck of cards? What is the probability of selecting a face card (Jack, Queen, or King) in a single draw from a deck of cards? Four cards are dealt from a deck with replacement. What is the probability that all four cards are aces? Solution Solution Solution 15 Joint and Marginal Probabilities A joint probability is the probability of two (or more) events happening together E.g. The probability that a person likes statistics and likes cats A marginal probability is the probability of just one of those events E.g. probability of liking statistics 16

5 Joint and Marginal Probabilities Likes Stat (A) Does Not Like Stats (not A) Marginal Likes Cats (B) Does Not Like Cats (not B) Marginal Conditional If two events are not independent of each other, then knowing whether one event occurred changes the probability that the other event might occur E.g., knowing that a person is an introvert decreases the probability that you will find the person in a social situation Conditional probabilities give the probability of one event given that another event has occurred 18 Conditional The conditional probability of event B occurring given that event A has occurred is given by: p(b A) = p(a and B) / p(a) The values of p(a and B) and p(a) can be easily gotten from the table of joint and marginal probabilities 19 Conditional What is the probability that a person likes cats given he or she likes statisitcs? p(likes cats likes stats) = p(likes cats and likes stats) / p(likes stats).167 /.222 =.75 Likes Stat (A) Does Not Like Stats (not A) Marginal Likes Cats (B) Does Not Like Cats (not B) Marginal

6 Conditional What is the probability that a person likes cats given he or she does not like statisitcs? p(likes cats dislikes stats) = p(likes cats and dislikes stats) / p(dislikes stats).556 /.778 =.71 Likes Stat (A) Does Not Like Stats (not A) Marginal Likes Cats (B) Does Not Like Cats (not B) Marginal The Multiplication Rule Revisited The multiplication rule given before applied only to two (or more) events that were independent of each other When the event are not independent, the multiplication rule must be revised to: p(a and B) = p(a) X p(b A) 22 Continuous Variables and When the variables / events are continuous rather than discrete, we can no longer simply count the occurrence of the event or count the total number of events The continuous nature of the variable implies that there is an infinite number of values that the variable can take on 23 of Continuous Events Rather than counting, the probability is determined by areas under a probability curve a b p(a X b) = area under curve between a and b / total area under curve 2

7 Unit Normal Curve The unit normal curve is frequently used in statistics to determine probabilities It is a normal (or Gaussian) curve with a standard deviation of 1 The area under the unit normal curve is 1 Many variables have approximately normal distributions 25 Determining Probabilities with the Unit Normal Distribution Weights are normally distributed What is the probability of a randomly selected female s weight being larger than 190 pounds? The mean weight of females is 133 pounds with a standard deviation of 22 pounds 26 Determining Probabilities with the Unit Normal Distribution Convert the raw score to a z-score: z = ( ) / 22 = 2.59 Use Table A in Appendix D of your text (or any other similar table) to find the area above a z- score of 2.59 z 27 Determining Probabilities with the Unit Normal Distribution The area above a z-score of 2.59 is.008 The total area under the unit normal curve is 1 The probability is.008 / 1 =.008 There are approximately 5 chances in 1000 of a randomly selected female weighing over 190 pounds z 28

8 Problems What is the probability of a randomly selected female weighing less than 100 pounds? What is the probability of a randomly selected female weighing more than 150 pounds or less than 110 pounds? What is the probability of two randomly selected females both weighing less than 90 pounds? Solution Solution Solution 29 The End 30 Draw a King How many kings are in a standard deck of cards? How many cards in a deck of cards? 52 The probability of drawing a king from a deck is given by # kings / # cards = / 52 = Problems 31 Draw a Face Card There are three types of face cards: Jacks, Queens, and Kings Each face card is mutually exclusive If a card is a Jack, it cannot be a Queen or King To determine the probability of one or more mutually exclusive events occurring, use the addition rule 32

9 Draw a Face Card p(jack or Queen or King) = p(jack) or p(queen) or p(king) From the previous problem, p(king) = p(jack) = p(queen) = p(king) p(jack or Queen or King) = = Draw a Face Card Another way of looking at this problem is simply to count the number of face cards in a deck There are 12 face cards in a deck p(face card) = # face cards / # cards in deck = 12 / 52 = The two answers agree Problems 3 Draw Four Aces The card that is dealt first has no influence on which card is dealt second, third, or fourth (because the card is replaced before the next draw) The card that is dealt second has no influence on which card is dealt first, third, or fourth, and so on Thus the draws are independent events 35 Draw Four Aces To determine the probability of several events happening together, use the multiplication rule p(ace on first draw and Ace on second draw and Ace on third draw and Ace on fourth draw) = p(ace on first draw) X p(ace on second draw) X p(ace on third draw) X p(ace on fourth draw) 36

10 Draw Four Aces p(ace on first draw) = # of aces / # cards There are four aces and 52 cards in the deck p(ace on first draw) = / 52 = The card is replaced and the deck is reshuffled p(ace on second draw) = p(ace on third draw) = p(ace on fourth draw) = / 52 = Draw Four Aces p(draw four aces) = X X X = This event would occur roughly 35 in 1,000,000 times Problems 38 Less Than 100 Pounds First, convert the raw score to a z-score N(133, 22), raw score = 100 z = ( ) / 22 = Draw a unit normal distribution and put the z-score on it z = Less Than 100 Pounds We want the area to the left of the z-score Because the unit normal distribution is symmetrical, the area to the left of z = -1.5 is the same as the area to the right of z = 1.5 z = -1.5 z = 1.5 0

11 Less Than 100 Pounds Find the area to the right of z = 1.5 Consult Table A or this table The area corresponds to.0668 Thus, 6.68% of randomly selected females should weigh less than 100 pounds z = -1.5 z = Less Than 100 Pounds Because the total area under the curve is 1, the area to the left of any z- score plus the area to the right of the z-score must equal 1 Because of symmetry, the area to the left of a negative z-score equals 1 - area to the left of the same, positive z-score z = Less Than 100 Pounds Area to the left of z equal to -1.5 equals 1 - area to the left of z equal =.0668 z = 1.5 Problems 3 > 150 or < 110 This problem calls for the addition rule of mutually exclusive events Determine the probability of a randomly selected female weighing more than 150 pounds Determine the probability of a randomly selected female weighing less than 110 pounds

12 > 150 Pounds Convert 150 pounds to a z-score z = / 22 = 0.77 Draw the distribution and z-score Consult a table to find the area above a z- score of.77 p(> 150) =.2206 z =.77 5 < 110 Pounds Convert 110 pounds to a z-score z = / 22 = Draw the distribution and z-score Consult a table to find the area below a z- score of p(< 110) =.169 z = p(>150 or <110) Combine the probabilities with the addition rule: p(>150 or <110) = p(>150) + p(<110) = =.3675 Roughly 37% of randomly selected women will weigh more than 150 pounds or less than 110 pounds Problems 7 Two Weighing Less than 90 Pounds This is an example of the multiplication rule for independent events p(a < 90 and B < 90) = p(a < 90) X p(b < 90) Determine the probability of randomly selecting one female weighing less than 90 pounds 8

13 < 90 Convert to a z-score: z = (90-133) / 22 = Draw the distribution and z-score Find, in a table, the area above z = 1.95 p(a < 90) =.0256 p(b < 90) =.0256 z = Combine Using Multiplication Rule p(a < 90 and B < 90) = p(a < 90) X p(b < 90) =.0256 X.0256 = Only about 66 times out of 100,000 times that you randomly selected two women would you expect both to weigh less than 90 pounds. Problems 50

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