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Recording numerical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Entering date and time data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Checking Your Entered Data for Errors. . . . . . . . . . . . . . . . . . . . . . . . . 109

Creating a File that Describes Your Data File . . . . . . . . . . . . . . . . . . . . 110

CHAPTER 9: Summarizing and Graphing Your Data. . . . . . . . . . . . . 111

Summarizing and Graphing Categorical Data. . . . . . . . . . . . . . . . . . . . 112

Summarizing Numerical Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Locating the center of your data. . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Describing the spread of your data . . . . . . . . . . . . . . . . . . . . . . . . . 119

Numerically expressing the symmetry and shape

of the distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Structuring Numerical Summaries into Descriptive Tables . . . . . . . . 123

Graphing Numerical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Showing the distribution with histograms. . . . . . . . . . . . . . . . . . . . 124

Summarizing grouped data with bars, boxes, and whiskers . . . . 126

Depicting the relationships between numerical variables

with other graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

CHAPTER 10: Having Confidence in Your Results. . . . . . . . . . . . . . . . . . 129

Feeling Confident about Confidence Interval Basics. . . . . . . . . . . . . . 130

Defining confidence intervals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Understanding and interpreting confidence levels. . . . . . . . . . . . 131

Taking sides with confidence intervals. . . . . . . . . . . . . . . . . . . . . . . 132

Calculating Confidence Intervals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Before you begin: Formulas for confidence limits in

large samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

The confidence interval around a mean . . . . . . . . . . . . . . . . . . . . . 134

The confidence interval around a proportion. . . . . . . . . . . . . . . . . 135

The confidence interval around an event count or rate. . . . . . . . 136

Relating Confidence Intervals and Significance Testing. . . . . . . . . . . . 137

PART 4: COMPARING GROUPS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

CHAPTER 11: Comparing Average Values between Groups. . . . . . 141

Grasping Why Different Situations Need Different Tests. . . . . . . . . . 142

Comparing the mean of a group of numbers to

a hypothesized value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

Comparing the mean of two groups of numbers. . . . . . . . . . . . . . 143

Comparing the means of three or more groups of numbers. . . . 143

Comparing means in data grouped on several

different variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Adjusting for a confounding variable when

comparing means. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Comparing means from sets of matched numbers. . . . . . . . . . . . 145

Comparing means of matched pairs . . . . . . . . . . . . . . . . . . . . . . . . 146