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Using the Calculator for Normal Distributions Standard Normal Go to 2nd Distribution Find #2 – Normalcdf Key stroke is Normalcdf(Begin, end) If standardized, then mean & st. deviation do not have to be given. Find P(1.1<z<1.89) Press: Normalcdf (1.1, 1.89) enter You should get 0.1063 Check By chart method: 0.9706 – 0.8643 = 0.1063 To represent infinity we use 1E99 (1X10^99) for positive infinity -1E99 (-1x10^99) for negative infinity Represent in notation with ∞ Find: P(z<2.07) Find: P(z>0.12) Try the following P(z < -1.23) P(-1.2<z<2.05) P(z>1.23) Average age of a student is 20 with standard deviation of 2.1 years. What’s the probability that a student’s age is more than 23? A professor can grade an average of 12 papers per day with a standard deviation of 1.8 papers. What is the probability that he can grade between 10 and 13 papers? The mean time to finish a test is 38 minutes with a standard deviation of 4.8 minutes. What’s the probability that a person takes more than 45 minutes to finish the test? The mean time to run a race is 6.7 minutes with a standard deviation of 0.37 minutes. Find the probability that it took less than between 6.5 and 6.8 minutes? Finding a percentile…. Use Invnorm(%) to get the z-score The mean score on a test is 70 with = 3. What’s the cutoff score for the 90th percentile? If the mean test score is 88 with standard deviation of 2, find the cutoff scores for the middle 40%. So if the mean is 34 and you know that 78% scored less than 36, what’s the standard deviation? If 28% scored less than 36 on a test, and 87% scored more than 34, what’s the mean and the standard deviation? Homework Worksheet