Major League Baseball now records information about every pitch thrown in every

Question

Major League Baseball now records information about every pitch thrown in every game of every season. Statistician Jim Albert compiled data about every pitch thrown by 20 starting pitchers during the 2009 MLB season. The data set included the type of pitch thrown (curveball, changeup, slider, etc.) as well as the speed of the ball as it left the pitcher’s hand. A histogram of speeds for all 30,740 four-seam fastballs thrown by these pitchers during the 2009 season is shown below, from which we can see that the speeds of these fastballs follow a Normal model with mean ? = 92.12 mph and a standard deviation of ? = 2.43 mph.

Compute the z-score of pitch with speed 94.4 mph. (Round answer to two decimal places.)



Find the probability that these four-seam fastballs that you would expect to have speeds between 91.5 mph and 97.3 mph? (Answer as a decimal, not a percent, and round to at least 3 decimal places.)



Find the probability that these four-seam fastballs that you would expect to have speeds above 97.3 mph? (Answer as a decimal, not a percent, and round to at least three decimal places.)



A baseball fan wishes to identify the four-seam fastballs among the slowest 16% of all such pitches. Below what speed must a four-seam fastball be in order to be included in the slowest 16%? (Answer to the nearest 0.1 mph.)

Explanation / Answer

a) mean = 92.12 , sd = 2.43
z= (X - mean)/sd = (X - 92.12)/2.43

z = (94.4 - 92.12)/2.43= 0.93827160493

b)
P( 91.5 <X < 97.3)
= P ( ?0.26<Z<2.13 )=0.586
c)

P(X > 97.3)
= P (Z>2.13)=0.0166

d)
P(X < x) = 0.16
P(Z < z) = 0.16
z = -0.994

X = 92.12 + z * 2.43
= 92.12 -0.994 *2.43
= 89.70458

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