Copyright © 2005 Pearson Education, Inc. Slide 6-1

Copyright © 2005 Pearson Education, Inc. Chapter 6

Copyright © 2005 Pearson Education, Inc. Slide 6-3 The mean is what we most commonly call the average value. It is defined as follows: mean = The median is the middle value in the sorted data set (or halfway between the two middle values if the number of values is even). The mode is the most common value (or group of values) in a distribution. Measures of Center in a Distribution 6-A

Copyright © 2005 Pearson Education, Inc. Slide 6-4 Middle Value for a Median 6-A (sorted list) (odd number of values) median is 6.44 exact middle

Copyright © 2005 Pearson Education, Inc. Slide 6-5 No Middle Value for a Median 6-A (sorted list) (even number of values) median is 5.02

Copyright © 2005 Pearson Education, Inc. Slide 6-6 Mode Examples 6-A a b c Mode is 5 Bimodal (2 and 6) No Mode

Copyright © 2005 Pearson Education, Inc. Slide 6-7 Symmetric and Skewed Distributions 6-A Mode = Mean = Median SYMMETRIC SKEWED LEFT (negatively) Mean Mode Median SKEWED RIGHT (positively) Mean Mode Median

Copyright © 2005 Pearson Education, Inc. Slide 6-8 Why Variation Matters 6-B Big Bank (three line wait times): Best Bank (one line wait times):

Copyright © 2005 Pearson Education, Inc. Slide 6-9 Five Number Summaries & Box Plots 6-B lower quartile = 5.6 high value (max) = 7.8 upper quartile = 7.7 median = 7.2 lower quartile = 6.7 low value (min) = 6.6 Best BankBig Bank high value (max) = 11.0 low value (min) = 4.1 upper quartile = 8.5 median = 7.2

Copyright © 2005 Pearson Education, Inc. Slide 6-10 Standard Deviation 6-B Let A = {2, 8, 9, 12, 19} with a mean of 10. Use the data set A above to find the sample standard deviation.

Copyright © 2005 Pearson Education, Inc. Slide 6-11 Range Rule of Thumb for Standard Deviation 6-B We can estimate standard deviation by taking an approximate range, (usual high – usual low), and dividing by 4. The reason for the 4 is due to the idea that the usual high is approximately 2 standard deviations above the mean and the usual low is approximately 2 standard deviations below. Going in the opposite direction, on the other hand, if we know the standard deviation we can estimate: low value  mean – 2 x standard deviation high value  mean + 2 x standard deviation

Copyright © 2010, 2007, 2004 Pearson Education, Inc. Outliers for Modified Boxplots For purposes of constructing modified boxplots, we can consider outliers to be data values meeting specific criteria. In modified boxplots, a data value is an outlier if it is... Data > Q  IQR Data < Q  IQR or

Copyright © 2010, 2007, 2004 Pearson Education, Inc. Modified Boxplots - Example Pulse rates of females listed in Data Set 1 in Appendix B.

Copyright © 2010, 2007, 2004 Pearson Education, Inc. Percentiles are measures of location. There are 99 percentiles denoted P 1, P 2,... P 99, which divide a set of data into 100 groups with about 1% of the values in each group.

Copyright © 2010, 2007, 2004 Pearson Education, Inc. Finding the Percentile of a Data Value Percentile of value x = 100 Do problems on page 125 number of values less than x total number of values

Copyright © 2005 Pearson Education, Inc. Slide 6-16 The Rule for a Normal Distribution 6-C (mean)

Copyright © 2005 Pearson Education, Inc. Slide % within 1 standard deviation of the mean 6-C (mean) ++ -- (just over two thirds) The Rule for a Normal Distribution

Copyright © 2005 Pearson Education, Inc. Slide % within 1 standard deviation of the mean 95% within 2 standard deviations of the mean 6-C (mean) ++ +2  -2  -- (most) The Rule for a Normal Distribution

Copyright © 2005 Pearson Education, Inc. Slide % within 1 standard deviation of the mean 95% within 2 standard deviations of the mean 6-C 99.7% within 3 standard deviations of the mean (mean) ++ +2  +3  -3  -2  -- (virtually all) The Rule for a Normal Distribution

Copyright © 2005 Pearson Education, Inc. Slide 6-20 Also known as the Empirical Rule 6-C

Copyright © 2005 Pearson Education, Inc. Slide 6-21 Z-Score Formula 6-C Example: If the nationwide ACT mean were 21 with a standard deviation of 4.7, find the z-score for a 30. What does this mean? This means that an ACT score of 30 would be about 1.91 standard deviations above the mean of 21.