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# Chapter_9 - Chapter 9 Gathering and Measuring Information...

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Chapter 9: Gathering and Measuring Information October 11, 2011

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Descriptive Statistics: Terms to Know Unit of observation: what or who is being described or analyzed or entity under study. It can be an event, person, or an object. Data points: individual observations that collectively make up the data we are trying to describe.
Descriptive Statistics: The Mean and the Median The mean or average is the sum of a set of numbers divided by the number of observations (also called n). The median is the midpoint in a distribution of numbers when you organize them from smallest to largest. Deciles, quartiles, and percentiles are ways to describe where an observation falls in a range of observations.

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Examples of Mean, Median, and Percentiles Percentiles -Over 50% of the students studied less than 2 hours. Hrs Studied Student 1 0 Student 2 1 Mean is 28/10 2.80 Student 3 1 Median 2 Student 4 2 Student 5 2 Student 6 2 Student 7 3 Student 8 3 Student 9 4 Student 10 10 Sum of Hrs 28
How do I know whether to use the mean or median? What is an outlier? A data point that lies far from the rest of the distribution. Outliers affect the mean but not the median. For example, in my Politics of Poverty class, one of student’s has a dad that makes \$750,000! That is an outlier compared to the class average of about \$125,000.

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Chapter_9 - Chapter 9 Gathering and Measuring Information...

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