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The Normal Probability Distribution Points of Inflection s m - 3s m - 2s m - s m m +s m + 2s m + 3s Main characteristics of the Normal Distribution • Bell Shaped, symmetric • Points of inflection on the bell shaped curve are at m – s and m + s. That is one standard deviation from the mean • Area under the bell shaped curve between m – s and m + s is approximately 2/3. • Area under the bell shaped curve between m – 2s and m + 2s is approximately 95%. • Close to 100% of the area under the bell shaped curve between m – 3s and m + 3s, There are many Normal distributions depending on by m and s Normal m = 100, s =20 0.03 Normal m = 100, s = 40 Normal m = 140, s =20 f(x) 0.02 0.01 0 0 50 100 x 150 200 The Standard Normal Distribution m = 0, s = 1 0.4 0.3 0.2 0.1 0 -3 -2 -1 0 1 2 3 • There are infinitely many normal probability distributions (differing in m and s) • Area under the Normal distribution with mean m and standard deviation s can be converted to area under the standard normal distribution • If X has a Normal distribution with mean m and standard deviation s than has a standard normal distribution z X -m s has a standard normal distribution. • z is called the standard score (z-score) of X. Converting Area under the Normal distribution with mean m and standard deviation s to Area under the standard normal distribution Perform the z-transformation z then X -m P a X b s Area under the Normal distribution with mean m and standard deviation s a - m X - m b - m P s s s b-m a - m P z s s Area under the standard normal distribution Area under the Normal distribution with mean m and standard deviation s P a X b s a m b Area under the standard normal distribution b-m a - m P z s s 1 a-m s 0 b-m s Using the tables for the Standard Normal distribution Example Find the area under the standard normal curve between z = - and z = 1.45 0.9265 0 • A portion of Table 3: z 0.00 0.01 0.02 0.03 1.45 0.04 z 0.05 .. . 1.4 .. . P( z 1.45) 0.9265 0.9265 0.06 Example Find the area to the left of -0.98; P(z < -0.98) Area asked for -0.98 0 P ( z < - 0.98) 0.1635 Example Find the area under the normal curve to the right of z = 1.45; P(z > 1.45) Area asked for 0.9265 0 1.45 P( z 1.45) 1.0000 - 0.9265 0.0735 z Example Find the area to the between z = 0 and of z = 1.45; P(0 < z < 1.45) 0 1.45 P( z < 1.45) 0.9265 - 0.5000 0.4265 • Area between two points = differences in two tabled areas z Notes Use the fact that the area above zero and the area below zero is 0.5000 the area above zero is 0.5000 When finding normal distribution probabilities, a sketch is always helpful Example: Find the area between the mean (z = 0) and z = -1.26 Area asked for - 1.26 0 z P( -1.26 < z < 0) 0.5000 - 0.1038 0.3962 Example: Find the area between z = -2.30 and z = 1.80 Required Area .-2.30 0 . 1.80 P( -1.26 < z < 1.80) 0.9641 - 0.0107 0.9534 Example: Find the area between z = -1.40 and z = -0.50 Area asked for -1.40 - 0.500 P( -1.40 < z < -0.50) 0.3085 - 0.0808 0.2277 Computing Areas under the general Normal Distributions (mean m, standard deviation s) Approach: 1. Convert the random variable, X, to its z-score. z X -m s 2. Convert the limits on random variable, X, to their z-scores. 3. Convert area under the distribution of X to area under the standard normal distribution. b-m a - m Pa X b P z s s Example Example: A bottling machine is adjusted to fill bottles with a mean of 32.0 oz of soda and standard deviation of 0.02. Assume the amount of fill is normally distributed and a bottle is selected at random: 1) Find the probability the bottle contains between 32.00 oz and 32.025 oz 2) Find the probability the bottle contains more than 31.97 oz Solutions part 1) When x 32.00 ; When x 32.025; 32.00 - m 32.00 - 32.0 0.00 z s 0.02 z 32.025 - m 32.025 - 32.0 1.25 s 0.02 Graphical Illustration: Area asked for 32.0 0 32.025 1.25 x z 32.0 - 32.0 X - 32.0 32.025 - 32.0 < < P ( 32.0 < X < 32.025) P 0.02 0.02 0.02 P ( 0 < z < 1.25) 0. 3944 Example, Part 2) 31.97 - 150 . 32.0 0 x z x - 32.0 3197 . - 32.0 P( z -150) P( x 3197 . ) P . 0.02 0.02 1.0000 - 0.0668 0.9332 Combining Random Variables Quite often we have two or more random variables X, Y, Z etc We combine these random variables using a mathematical expression. Important question What is the distribution of the new random variable? An Example Suppose that a student will take three tests in the next three days 1. Mathematics (X is the score he will receive on this test.) 2. English Literature (Y is the score he will receive on this test.) 3. Social Studies (Z is the score he will receive on this test.) Assume that 1. X (Mathematics) has a Normal distribution with mean m = 90 and standard deviation s = 3. 2. Y (English Literature) has a Normal distribution with mean m = 60 and standard deviation s = 10. 3. Z (Social Studies) has a Normal distribution with mean m = 70 and standard deviation s = 7. Graphs 0.14 X (Mathematics) m = 90, s = 3. 0.12 0.1 0.08 Z (Social Studies) m = 70 , s = 7. 0.06 0.04 Y (English Literature) m = 60, s = 10. 0.02 0 0 20 40 60 80 100 Suppose that after the tests have been written an overall score, S, will be computed as follows: S (Overall score) = 0.50 X (Mathematics) + 0.30 Y (English Literature) + 0.20 Z (Social Studies) + 10 (Bonus marks) What is the distribution of the overall score, S? Sums, Differences, Linear Combinations of R.V.’s A linear combination of random variables, X, Y, . . . is a combination of the form: L = aX + bY + … where a, b, etc. are numbers – positive or negative. Most common: Sum = X + Y Difference = X – Y Others Averages = 1/3 X + 1/3 Y + 1/3 Z Weighted averages = 0.40 X + 0.25 Y + 0.35 Z Means of Linear Combinations If L = aX + bY + … The mean of L is: Mean(L) = a Mean(X) + b Mean(Y) + … mL = a mX + b mY + … Most common: Mean( X + Y) = Mean(X) + Mean(Y) Mean(X – Y) = Mean(X) – Mean(Y) Variances of Linear Combinations If X, Y, . . . are independent random variables and L = aX + bY + … then Variance(L) = a2 Variance(X) + b2 Variance(Y) + … s L2 a 2s X2 + b2s Y2 + Most common: Variance( X + Y) = Variance(X) + Variance(Y) Variance(X – Y) = Variance(X) + Variance(Y) Combining Independent Normal Random Variables If X, Y, . . . are independent normal random variables, then L = aX + bY + … is normally distributed. In particular: X + Y is normal with mean m X + mY standard deviation s X2 + s Y2 X – Y is normal with mean m X - mY standard deviation s X2 + s Y2 Example: Suppose that one performs two independent tasks (A and B): X = time to perform task A (normal with mean 25 minutes and standard deviation of 3 minutes.) Y = time to perform task B (normal with mean 15 minutes and std dev 2 minutes.) X and Y independent so T = X + Y = total time is normal mean m 25 + 15 40 with standard deviation s 32 + 22 3.6 What is the probability that the two tasks take more than 45 minutes to perform? 45 - 40 PT 45 P Z PZ 1.39 .0823 3.6 The distribution of averages (the mean) • Let x1, x2, … , xn denote n independent random variables each coming from the same Normal distribution with mean m and standard deviation s. n • Let x x i 1 n i 1 1 x1 + x2 + n n What is the distribution of x ? 1 + xn n The distribution of averages (the mean) Because the mean is a “linear combination” 1 1 m x m x1 + m x2 + n n 1 1 m + m + n n 1 + m xn n 1 1 + m n m m n n and 2 2 2 1 2 1 2 1 2 s s x1 + s x2 + + s xn n n n 2 2 2 s2 s2 1 2 1 2 1 2 s + s + + s n 2 n n n n n 2 x Thus if x1, x2, … , xn denote n independent random variables each coming from the same Normal distribution with mean m and standard deviation s. Then n x x i i 1 n 1 1 x1 + x2 + n n 1 + xn n has Normal distribution with mean m x m and variance s x2 s2 n standard deviation s x s n Example • Suppose we are measuring the cholesterol level of men age 60-65 • This measurement has a Normal distribution with mean m = 220 and standard deviation s = 17. • A sample of n = 10 males age 60-65 are selected and the cholesterol level is measured for those 10 males. • x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, are those 10 measurements Find the probability distribution of x ? Compute the probability that x is between 215 and 225 Example • Suppose we are measuring the cholesterol level of men age 60-65 • This measurement has a Normal distribution with mean m = 220 and standard deviation s = 17. • A sample of n = 10 males age 60-65 are selected and the cholesterol level is measured for those 10 males. • x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, are those 10 measurements Find the probability distribution of x ? Compute the probability that x is between 215 and 225 Solution Find the probability distribution of x Normal with m x m 220 s 17 and s x 5.376 n 10 P 215 x 225 215 - 220 x - 220 225 - 220 P 5.376 5.376 5.376 P -0.930 z 0.930 0.648 Graphs 0.08 The probability distribution of the mean 0.06 0.04 The probability distribution of individual observations 0.02 0 150 170 190 210 230 250 270 290 310 Normal approximation to the Binomial distribution Using the Normal distribution to calculate Binomial probabilities Binomial distribution n = 20, p = 0.70 0.2500 Approximating Normal distribution 0.2000 m np 14 s npq 2.049 0.1500 Binomial distribution 0.1000 0.0500 -0 -0.5 2 4 6 8 10 12 14 16 18 20 Normal Approximation to the Binomial distribution PX a Pa - 12 Y a + 12 • X has a Binomial distribution with parameters n and p • Y has a Normal distribution m np s npq 1 2 continuity correction 0.2500 Approximating Normal distribution 0.2000 P[X = a] 0.1500 Binomial distribution 0.1000 0.0500 -0 -0.5 2 4 6 8 10 a - 12 12 a 14 a+ 16 1 2 18 20 0.2500 0.2000 Pa - 12 Y a + 12 0.1500 0.1000 0.0500 -- -0.5 a 0.2500 0.2000 P[X = a] 0.1500 0.1000 0.0500 -- -0.5 a Example • X has a Binomial distribution with parameters n = 20 and p = 0.70 We want PX 13 The exact valu e PX 13 20 13 7 0.70 0.30 0.1643 13 Using the Normal approximation to the Binomial distribution PX 13 P12 12 Y 13 12 Where Y has a Normal distribution with: m np 20(0.70) 14 s npq 20.70.30 2.049 Hence P12.5 Y 13.5 12.5 - 14 Y - 14 13.5 - 14 P 2 . 049 2 . 049 2 . 049 P- 0.73 Z -0.24 = 0.4052 - 0.2327 = 0.1725 Compare with 0.1643 Normal Approximation to the Binomial distribution Pa X b p(a) + p(a + 1) + + p(b) 1 1 P a - 2 Y b + 2 • X has a Binomial distribution with parameters n and p • Y has a Normal distribution m np s npq 1 2 continuity correction 0.2500 Pa X b 0.2000 0.1500 0.1000 0.0500 -- -0.5 a - 12 a b b + 12 0.2500 Pa - 12 Y b + 12 0.2000 0.1500 0.1000 0.0500 -- -0.5 a - 12 a b b + 12 Example • X has a Binomial distribution with parameters n = 20 and p = 0.70 We want P11 X 14 The exact valu e P11 X 14 p(11) + p(12) + p(13) + p(14) 20 20 11 9 14 6 0.70 0.30 + + 0.70 0.30 11 14 0.0654 + 0.1144 + 0.1643 + 0.1916 0.5357 Using the Normal approximation to the Binomial distribution P11 X 14 P10 12 Y 14 12 Where Y has a Normal distribution with: m np 20(0.70) 14 s npq 20.70.30 2.049 Hence P10.5 Y 14.5 10.5 - 14 Y - 14 14.5 - 14 P 2 . 049 2 . 049 2 . 049 P-1.71 Z 0.24 = 0.5948 - 0.0436 = 0.5512 Compare with 0.5357 Comment: • The accuracy of the normal appoximation to the binomial increases with increasing values of n Example • The success rate for an Eye operation is 85% • The operation is performed n = 2000 times Find 1. The number of successful operations is between 1650 and 1750. 2. The number of successful operations is at most 1800. Solution • X has a Binomial distribution with parameters n = 2000 and p = 0.85 We want P1680 X 1720 P1679.5 Y 1720.5 where Y has a Normal distribution with: m np 2000(0.85) 1700 s npq 200.85.15 15.969 Hence P1680 X 1720 P1679.5 Y 1720.5 1679.5 - 1700 Y - 1700 1720.5 - 1700 P 15 . 969 15 . 969 15 . 969 P-1.28 Z 1.28 = 0.9004 - 0.0436 = 0.8008 Solution – part 2. We want PX 1800 PY 1800.5 Y - 1700 1800.5 - 1700 P 15 . 969 15 . 969 PZ 6.29 = 1.000