The Standard Deviation as a Ruler and the Normal Model
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1 The Standard Deviation as a Ruler and the Normal Model Al Nosedal University of Toronto Summer 2017 Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
2 My momma always said: Life was like a box of chocolates. You never know what you re gonna get. Forrest Gump. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
3 Simple Example Random Experiment: Rolling a fair die 300 times. Class Expected Frequency Expected Relative Freq 0 < x /6 1 < x /6 2 < x /6 3 < x /6 4 < x /6 5 < x /6 Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
4 Histogram of Expected Frequencies Histogram of expected frequencies frequency Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
5 Histogram of Expected Relative Frequencies Histogram of expected relative frequencies frequency Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
6 Density Curve A density curve is a curve that is always on or above the horizontal axis, and has area exactly 1 underneath it. A density curve describes the overall pattern of a distribution. The area under the curve and above any range of values is the proportion of all observations that fall in that range. Note. No set of real data is exactly described by a density curve. The curve is an idealized description that is easy to use and accurate enough for practical use. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
7 Accidents on a bike path Examining the location of accidents on a level, 5-mile bike path shows that they occur uniformly along the length of the path. The figure below displays the density curve that describes the distribution of accidents. a) Explain why this curve satisfies the two requirements for a density curve. b) The proportion of accidents that occur in the first mile of the path is the area under the density curve between 0 miles and 1 mile. What is this area? c) There is a stream alongside the bike path between the 0.8-mile mark and the 1.3-mile mark. What proportion of accidents happen on the bike path alongside the stream? d) The bike path is a paved path through the woods, and there is a road at each end. What proportion of accidents happen more than 1 mile from either road? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
8 Density Curve Density Curve Distance along bike path (miles) Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
9 Solution a) It is on or above the horizontal axis everywhere, and because it forms a 1/5 5 rectangle, the area beneath the curve is 1. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
10 Solution b) Density Curve proportion = 1 x 0.20 = Distance along bike path (miles) Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
11 Solution c) Density Curve proportion = ( ) x 0.20 = Distance along bike path (miles) Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
12 Solution d) Density Curve proportion = (4-1) x 0.20 = Distance along bike path (miles) Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
13 Normal Distributions A Normal Distribution is described by a Normal density curve. Any particular Normal distribution is completely specified by two numbers, its mean µ and standard deviation σ. The mean of a Normal distribution is at the center of the symmetric Normal curve. The standard deviation is the distance from the center to the change-of-curvature points on either side. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
14 Standard Normal Distribution Normal Distribution mean=0 and standard deviation= Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
15 Two Different Standard Deviations std. dev.= 2 std. dev.= Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
16 Two Different Means mean = -5 mean = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
17 The rule In the Normal distribution with mean µ and standard deviation σ: Approximately 68% of the observations fall within σ of the mean µ. Approximately 95% of the observations fall within 2σ of µ. Approximately 99.7% of the observations fall within 3σ of µ. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
18 Problem The national average for the verbal portion of the College Boards Scholastic Aptitude Test (SAT) is 507. The College Board periodically rescales the test scores such that the standard deviation is approximately 100. Answer the following questions using a bell-shaped distribution and the empirical rule for the verbal test scores. a. What percentage of students have an SAT verbal score greater than 607? b. What percentage of students have an SAT verbal score greater than 707? c. What percentage of students have an SAT verbal score between 407 and 507? d. What percentage of students have an SAT verbal score between 307 and 707? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
19 Solution a) % 68 % 16 % SAT score Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
20 Solution b) % 95 % 2.5 % SAT score Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
21 Solution c) % 34 % 34 % 16 % SAT score Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
22 Solution d) % 95 % 2.5 % SAT score Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
23 Fruit flies The common fruit fly Drosophila melanogaster is the most studied organism in genetic research because it is small, easy to grow, and reproduces rapidly. The length of the thorax (where the wings and legs attach) in a population of male fruit flies is approximately Normal with mean millimeters (mm) and standard deviation mm. Draw a Normal curve on which this mean and standard deviation are correctly located. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
24 Solution Thorax length Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
25 Fruit flies The lenght of the thorax in a population of male fruit flies is approximately Normal with mean mm and standard deviation mm. Use the rule to answer the following questions. a) What range of lengths covers almost all (99.7%) of this distribution? b) What percent of male fruit flies have a thorax length exceeding mm? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
26 Solution a) Between mm and mm (0.078)= % (0.078)= Thorax length Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
27 Solution b) 16% of thorax lenghts exceed mm % 16 % Thorax length Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
28 Monsoon rains The summer monsoon brings 80% of India s rainfall and is essential for the country s agriculture. Records going back more than a century show that the amount of monsoon rainfall varies from the year according to a distribution that is approximately Normal with mean 582 mm and standard deviation 82 mm. Use the rule to answer the following questions. a) Between what values do the monsoon rains fall in 95% of all years? b) How small are the monsoon rains in the dryest 2.5% of all years? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
29 Solution a) In 95% of all years, monsoon rain levels are between 582-2(82) and (82) i.e. 418 mm and 746 mm. b) The driest 2.5% of monsoon rainfalls are less than 418 mm; this is more than two standard deviations below the mean. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
30 Standard Normal Distribution The standard Normal distribution is the Normal distribution N(0, 1) with mean 0 and standard deviation 1. If a variable x has any Normal distribution N(µ,σ) with mean µ and standard deviation σ, then the standardized variable z = x µ σ has the standard Normal distribution. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
31 SAT vs ACT In 2010, when she was a high school senior, Alysha scored 670 on the Mathematics part of the SAT. The distribution of SAT Math scores in 2010 was Normal with mean 516 and standard deviation 116. John took the ACT and scored 26 on the Mathematics portion. ACT Math scores for 2010 were Normally distributed with mean 21.0 and standard deviation 5.3. Find the standardized scores for both students. Assuming that both tests measure the same kind of ability, who had the higher score? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
32 Solution Alysha s standardized score is z A = John s standardized score is = z J = = Alysha s score is relatively higher than John s. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
33 Men s and women s heights The heights of women aged 20 to 29 are approximately Normal with mean 64.3 inches and standard deviation 2.7 inches. Men the same age have mean height 69.9 inches with standard deviation 3.1 inches. What are the z-scores for a woman 6 feet tall and a man 6 feet tall? Say in simple language what information the z-scores give that the original nonstandardized heights do not. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
34 Solution We need to use the same scale, so recall that 6 feet = 72 inches. A woman 6 feet tall has standardized score z W = 2.7 (quite tall, relatively). A man 6 feet tall has standardized score = 2.85 z M = = Hence, a woman 6 feet tall is 2.85 standard deviations taller than average for women. A man 6 feet tall is only 0.68 standard deviations above average for men. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
35 Using the Normal table Use table A to find the proportion of observations from a standard Normal distribution that satisfies each of the following statements. In each case, sketch a standard Normal curve and shade the area under the curve that is the answer to the question. a) z < 1.42 b) z > 1.42 c) z < 2.35 d) 1.42 < z < 2.35 Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
36 Solution a) z* = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
37 Solution b) z* = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
38 Solution c) z* = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
39 Solution d) = = z* = z* = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
40 Monsoon rains The summer monsoon rains in India follow approximately a Normal distribution with mean 852 mm of rainfall and standard deviation 82 mm. a) In the drought year 1987, 697 mm of rain fell. In what percent of all years will India have 697 mm or less of monsoon rain? b) Normal rainfall means within 20% of the long-term average, or between 683 and 1022 mm. In what percent of all years is the rainfall normal? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
41 Solution a) 1. State the problem. Let x be the monsoon rainfall in a given year. The variable x has the N(852, 82) distribution. We want the proportion of years with x Standardize. Subtract the mean, then divide by the standard deviation, to turn x into a standard Normal z. Hence x 697 corresponds to z = Use the table. From Table A, we see that the proportion of observations less than 1.89 is Thus, the answer is 2.94%. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
42 Solution b) 1. State the problem. Let x be the monsoon rainfall in a given year. The variable x has the N(852, 82) distribution. We want the proportion of years with 683 < x < Standardize. Subtract the mean, then divide by the standard deviation, to turn x into a standard Normal z. 683 < x < 1022 corresponds to < z < , or 2.06 < z < Use the table. Hence, using Table A, the area is = 96.11%. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
43 The Medical College Admission Test Almost all medical schools in the United States require students to take the Medical College Admission Test (MCAT). The exam is composed of three multiple-choice sections (Physical Sciences, Verbal Reasoning, and Biological Sciences). The score on each section is converted to a 15-point scale so that the total score has a maximum value of 45. The total scores follow a Normal distribution, and in 2010 the mean was 25.0 with a standard deviation of 6.4. There is little change in the distribution of scores from year to year. a) What proportion of students taking the MCAT had a score over 30? b) What proportion had scores between 20 and 25? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
44 Solution a) 1. State the problem. Let x be the MCAT score of a randomly selected student. The variable x has the N(25, 6.4) distribution. We want the proportion of students with x > Standardize. Subtract the mean, then divide by the standard deviation, to turn x into a standard Normal z. Hence x > 30 corresponds to z > = Use the table. From Table A, we see that the proportion of observations less than 0.78 is Hence, the answer is = , or 21.77%. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
45 Solution b) 1. State the problem. Let x be the MCAT score of a randomly selected student. The variable x has the N(25, 6.4) distribution. We want the proportion of students with 20 x Standardize. Subtract the mean, then divide by the standard deviation, to turn x into a standard Normal z. 20 x 25 corresponds to z , or 0.78 z Use the table. Using Table A, the area is = , or 28.33%. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
46 Using a table to find Normal proportions Step 1. State the problem in terms of the observed variable x. Draw a picture that shows the proportion you want in terms of cumulative proportions. Step 2. Standardize x to restate the problem in terms of a standard Normal variable z. Step 3. Use Table A and the fact that the total are under the curve is 1 to find the required area under the standard Normal curve. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
47 Table A Use Table A to find the value z of a standard Normal variable that satisfies each of the following conditions. (Use the value of z from Table A that comes closest to satisfying the condition.) In each case, sketch a standard Normal curve with your value of z marked on the axis. a) The point z with 15% of the observations falling below it. b) The point z with with 70% of the observations falling above it. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
48 Solution a) z* = z* = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
49 Solution b) z* = = z* = Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
50 The Medical College Admission Test The total scores on the Medical College Admission Test (MCAT) follow a Normal distribution with mean 25.0 and standard deviation 6.4. What are the median and the first and third quartiles of the MCAT scores? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
51 Solution: Finding the median Because the Normal distribution is symmetric, its median and mean are the same. Hence, the median MCAT score is 25. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
52 Solution: Finding Q 1 1. State the problem. We want to find the MCAT score x with area 0.25 to its left under the Normal curve with mean µ = 25 and standard deviation σ = Use the table. Look in the body of Table A for the entry closest to It is This is the entry corresponding to z = So z = 0.67 is the standardized value with area 0.25 to its left. 3. Unstandardize to transform the solution from the z back to the original x scale. We know that the standardized value of the unknown x is z = So x itself satisfies x Solving this equation for x gives x = 25 + ( 0.67)(6.4) = = 0.67 Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
53 Solution: Finding Q 3 1. State the problem. We want to find the MCAT score x with area 0.75 to its left under the Normal curve with mean µ = 25 and standard deviation σ = Use the table. Look in the body of Table A for the entry closest to It is This is the entry corresponding to z = So z = 0.67 is the standardized value with area 0.75 to its left. 3. Unstandardize to transform the solution from the z back to the original x scale. We know that the standardized value of the unknown x is z = So x itself satisfies x Solving this equation for x gives x = 25 + (0.67)(6.4) = = 0.67 Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
54 Finding a value when given a proportion 1. State the problem. 2. Use the table. 3. Unstandardize to transform the solution from the z back to the original x scale. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
55 Example The summer monsoon rains in India follow approximately a Normal distribution with mean 852 mm of rainfall and standard deviation 82 mm. a) In the drought year 1987, 697 mm of rain fell. In what percent of all years will India have 697 mm or less of monsoon rain? b) Normal rainfall means within 20% of the long-term average, or between 683 and 1022 mm. In what percent of all years is the rainfall normal? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
56 R code a) Just type the following: pnorm(697, mean = 852, sd = 82); # You should get ; b) Just type the following: pnorm(1022, 852, 82) - pnorm(683, 852, 82); # You should get ; Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
57 Example Consider an investment whose return is Normally distributed with a mean of 10% and a standard deviation of 5%. a. Determine the probability of losing money. b. Find the probability of losing money when the standard deviation is equal to 10%. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
58 Solution a) X = return. We know that X is Normally distributed with mean (µ) 0.10 and standard deviation (σ) Another way of writing this is: X has a N(0.10, 0.05) distribution. a. P(X < 0) = P ( X µ σ < ) = P(Z < 2) = Therefore, the probability of losing money is 2.28%. (Using the Empirical Rule gives 2.5%) Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
59 Solution b) (If we increase the standard deviation to 10%). Y = return. We know that Y is Normally distributed with mean (µ) 0.10 and standard deviation (σ) Another way of writing this is: Y has a N(0.10, 0.10) distribution. ( ) a. P(Y < 0) = P Y µ σ < = P(Z < 1) = Therefore, the probability of losing money is 15.87%. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
60 Example The demand for a daily newspaper at a newsstand at a busy intersection is known to be Normally distributed with a mean of 150 and a standard deviation of 25. How many newspapers should the newsstand operator order to ensure that he runs short on no more than 20% of days? Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
61 Solution D = demand. We know that D has a Normal distribution with mean 150 and standard deviation State the problem. We want to find a number of newspapers, n, with area 0.80 to its left under the Normal curve with mean µ = 150 and standard deviation σ = Use the table. Look in the body of Table A for the entry closest to It is This is the entry corresponding to z = So z = 0.84 is the standardized value with area 0.80 to its left. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
62 Solution 3. Unstandardize to transform the solution from the z back to the original D scale. We know that the standardized value of the unknown n is z = z = So n itself satisfies n 150 = Solving this equation for x gives n = (0.84)(25) = 171 The newsstand operator should order 171 newspapers. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
63 Example Mensa is an organization whose members possess IQs that are in the top 2% of the population. It is known that IQs are Normally distributed with a mean of 100 and a standard deviation of 16. Find the minimum IQ needed to be a Mensa member. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
64 Solution IQ = µ + z σ = (16) = We could round it to 133. Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
65 R Code qnorm(0.98, mean = 100, sd = 16); Al Nosedal University of Toronto The Standard Deviation as a Ruler and the Normal Model Summer / 65
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