3.2 Area under the curve, Part II.

What percent of a standard normal distribution N(µ = 0, SD = 1) is found in each region? Be sure to draw a graph.

  1. Z < -1.35
visualize.norm(stat=-1.35,mu=0,sd=1,section="lower")

  1. Z > 1.48
visualize.norm(stat=1.48,mu=0,sd=1,section="upper")

  1. 0.4<Z< 1.5
visualize.norm(stat=c(0.4,1.5),mu=0,sd=1,section="bounded")

  1. |Z| > 2

Z >2 and -Z >2 or Z < -2

Finding probability of -2 > Z > 2

visualize.norm(stat=c(-2,2),mu=0,sd=1,section="bounded")

#3.4 Triathlon times, Part I.

In triathlons, it is common for racers to be placed into age and gender groups. Friends Leo and Mary both completed the Hermosa Beach Triathlon, where Leo competed in the Men, Ages 30 - 34 group while Mary competed in the Women, Ages 25 - 29 group. Leo completed the race in 1:22:28 (4948 seconds), while Mary completed the race in 1:31:53 (5513 seconds). Obviously Leo finished faster, but they are curious about how they did within their respective groups. Can you help them? Here is some information on the performance of their groups: • The finishing times of the Men, Ages 30 - 34 group has a mean of 4313 seconds with a standard deviation of 583 seconds. • The finishing times of the Women, Ages 25 - 29 group has a mean of 5261 seconds with a standard deviation of 807 seconds. • The distributions of finishing times for both groups are approximately Normal. Remember: a better performance corresponds to a faster finish.

  1. Write down the short-hand for these two normal distributions.
#Men Ages 30-40: 
men_mean=4313
men_sd=583 

#Women Ages 30-34: 
women_mean=5261 
women_sd=807
  1. What are the Z-scores for Leo’s and Mary’s finishing times? What do these Z-scores tell you?
Z_score_Leo<-(4948-men_mean)/men_sd
Z_score_Leo
## [1] 1.089194
Z_score_Mary<-(5513-women_mean)/women_sd
Z_score_Mary
## [1] 0.3122677

Z-score is the number of standard deviations from the mean.

Leo’s result is 1.09 standard deviations from the mean while Mary’s results is 0.31 standard deviations from the mean.

  1. Did Leo or Mary rank better in their respective groups? Explain your reasoning.

Mary ranks better since her result is closer to the median(since mean and median are equal in normal distribution) than Leo’s.

  1. What percent of the triathletes did Leo finish faster than in his group?
1-pnorm(4948, men_mean, men_sd)
## [1] 0.1380342

Leo finished faster than 13.8% of the triathletes.

Let’s draw a graph.

visualize.norm(Z_score_Leo,mu=men_mean,sd=583,section="lower")

  1. What percent of the triathletes did Mary finish faster than in her group?
Z_score_Leo<-(4948-4313)/583
Z_score_Leo
## [1] 1.089194
Z_score_Mary<-(5513-5261)/807
Z_score_Mary
## [1] 0.3122677
  1. If the distributions of finishing times are not nearly normal, would your answers to parts (b) - (e) change? Explain your reasoning.