CHAPTER 6 PART 1 THE STANDARD DEVIATION AS A RULER
The men’s combined skiing event in the winter Olympics consists of two races: a downhill and a slalom. Times for the two events are added together and the skier with the lowest total time wins. In the 2006 Winter Olympics, the mean slalom time was seconds with a standard deviation of seconds. The mean downhill time was seconds with a standard deviation of seconds. Ted Ligety of the United States, who won the gold medal with a combined time of seconds, skied the slalom in seconds and the downhill in seconds. On which race did he do better compared with the competition? How would you answer the following question?
Standardizing values is useful in comparing values from different sets of data. We give the values z- scores (standardized scores) for comparisons.
Example: Find the z-score for 95 if the data has a mean of 90 and a standard deviation of 3. This means that 95 is 1.67 standard deviations away from the mean.
A positive z-score means the value is greater than the mean. A negative z-score means the value is less than the mean. A z-score of 0 means the value is equal to the mean.
Now, back to the question… On which race did Ted Ligety do better? Slalom: time = sec; mean = sec; SD = sec Downhill: time = sec; mean = sec; SD = sec
Which z-score is better? z = -1.2 or z = -0.21? Context is important. Since we are looking at race times, the lower the better. The time for the slalom was 1.2 standard deviations below the average, while the time for the downhill was only 0.21 standard deviations below the average. So the better performance was in the slalom.
In the 2006 winter Olympics men’s combined event, Ivica Kostelic of Croatia skied the slalom in seconds and the downhill in seconds. He beat Ted Ligety in the downhill, but not in the slalom. Who should have won the gold medal? Calculate the z-scores for Ivica Kostelic (slalom mean = and SD = sec; downhill mean = and SD = sec) Compare the sum of the z-scores Practice:
Ted Ligety had a z-score sum of compared to Ivica Kostelic with a z-score sum of Kostelic’s total is better. Because the standard deviation of the downhill times is so much smaller, Kostelic’s better performance there means that he would have won the event if standardized scores were used. Ligety won the gold, but perhaps Kostelic should have.
Your Statistics teacher has announced that the lower of your two tests will be dropped. You got a 90 on test 1 and an 80 on test 2. You’re all set to drop the 80 until she announces that she grades on a curve. She standardized the scores in order to decide which is the lower one. If the mean on the first test was 88 with a standard deviation of 4 and the mean on the second was 75 with a standard deviation of 5, which one will be dropped?
If the mean on the first test was 88 and the standard deviation was 4, which test score would be needed to have a z-score of 1.5?
Today’s Assignment: Read Chapter 6 Homework: page 129 #8-16