Section 3.4 The Normal Distribution Properties of the Normal Distribution Curve 1. We denote the normal random variable with X = x. 2. The curve has a peak at x = µ. 3. The curve is symmetric about the line x = µ. 4. The curve always lies above the x-axis but approaches the x-axis as x extends indefinitely in either direction. 5. The area under the curve is 1. 6. The Standard Normal distribution has µ = 0 and σ = 1. We denote the standard normal random variable with Z = z. Calculating the Probability of a Normal Random Variable The probability P (a < X < b) that X lies between a and b is the area under the curve between x = a and x = b. This can be found using probability tables but in this class we will use the calculator function normalcdf to calculate probabilities for a Normal random variable. Calculator Steps: Click 2ND, VARS, 2. You should see normalcdf( on your screen. The format is normalcdf(smallest x-value/z-value, biggest x-value/z-value, µ, σ). Use E99 if the biggest x-value/z-value is and E99 if the smallest x-value/z-value is. To get E99 click 2ND,,. Note: We never use normalpdf in this class.
1. Answer the following: (a) Choose a sketch of the area under the standard normal curve corresponding to P (0.29 < Z < 1). (a) (b) (c) (d) (b) Find the value of the probability of the standard normal variable Z corresponding to P (0.29 < Z < 1). (Give answer to four decimal places.) 2. Find the indicated probability given that Z is a random variable with a standard normal distribution. (Round answer to four decimal places.) P (Z 0.71) 3. Suppose X is a normal random variable with µ = 376 and σ = 17. Find the following probabilities. (Give answers to four decimal places.) (a) P (X < 409) 2 Spring 2018, Maya Johnson
(b) P (389 < X < 411) (c) P (X > 409) Inverse Normal Distribution: Suppose we are given the probability or area under the curve and are asked to find the random variable value that corresponds to the given probability. To solve this problem we will use the calculator function invnorm. Calculator Steps: Click 2ND, VARS, 3. You should see invnorm( on your screen. The format is invnorm(probability to the left of X = x or Z = z, µ, σ). 4. Let Z be the standard normal variable. Find the values of a that satisfy the given probabilities. (Give answers to four decimal places.) (a) P (Z > a) = 0.2829 (b) P ( a < Z < a) = 0.4441 3 Spring 2018, Maya Johnson
(c) P (Z < a) = 0.2441 5. Find the indicated quantities given that X is a normal random variable with a mean of 40 and a standard deviation of 10. (Round answers to four decimal places.) (a) Find the value of b such that P (X b) = 0.1515 (b) Find the values of A and B such that P (A X B) = 0.9146 if A and B are symmetric about the mean. 4 Spring 2018, Maya Johnson
6. On the average, a student takes 119 words/minute midway through an advanced court reporting course at the American Institute of Court Reporting. Assuming that the dictation speeds of the students are normally distributed and that the standard deviation is 24 words/minute, find the probability that a student randomly selected from the course can make dictation at the following speeds. (Give answers to four decimal places.) (a) more than 167 words/minute (b) between 143 and 167 words/minute (c) less than 71 words/minute 7. The weight of topsoil sold in a week is normally distributed with a mean of 800 tons and a standard deviation of 32 tons. (Round answers to two decimal places.) (a) What percentage of weeks will sales exceed 864 tons? (b) What percentage of weeks will sales be less than 784 tons? (c) What percentage of weeks will sales be between 752 and 816 tons? 5 Spring 2018, Maya Johnson
8. A teacher wishes to curve a test whose grades were normally distributed with a mean of 60 and standard deviation of 15. The top 10% of the class will get an A, the next 30% of the class will get a B, the next 35% of the class will get a C, the next 20% of the class will get a D and the bottom 5% of the class will get an F. Find the cutoff for each of these grades. (Round answers to two decimal places.) (a) The A cutoff is a grade of (b) The B cutoff is a grade of (c) The C cutoff is a grade of (d) The D cutoff is a grade of 6 Spring 2018, Maya Johnson