x Biostatistics For Dummies

PART 5: LOOKING FOR RELATIONSHIPS WITH

CORRELATION AND REGRESSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

CHAPTER 15: Introducing Correlation and Regression. . . . . . . . . . . 201

Correlation: Estimating How Strongly Two Variables

Are Associated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

Lining up the Pearson correlation coefficient. . . . . . . . . . . . . . . . . 202

Analyzing correlation coefficients. . . . . . . . . . . . . . . . . . . . . . . . . . . 203

Regression: Discovering the Equation that Connects

the Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Understanding the purpose of regression analysis. . . . . . . . . . . . 207

Talking about terminology and mathematical notation . . . . . . . . 208

Classifying different kinds of regression. . . . . . . . . . . . . . . . . . . . . 209

CHAPTER 16: Getting Straight Talk on Straight-Line

Regression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

Knowing When to Use Straight-Line Regression. . . . . . . . . . . . . . . . . . 213

Understanding the Basics of Straight-Line Regression. . . . . . . . . . . . 215

Running a Straight-Line Regression. . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Taking a few basic steps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

Walking through an example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

Interpreting the Output of Straight-Line Regression. . . . . . . . . . . . . . 220

Seeing what you told the program to do. . . . . . . . . . . . . . . . . . . . . 220

Evaluating residuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

Making your way through the regression table. . . . . . . . . . . . . . . 224

Wrapping up with measures of goodness-of-fit. . . . . . . . . . . . . . . 227

Scientific fortune-telling with the prediction formula . . . . . . . . . . 228

Recognizing What Can Go Wrong with Straight-Line Regression. . . . 229

Calculating the Sample Size You Need. . . . . . . . . . . . . . . . . . . . . . . . . . 230

CHAPTER 17: More of a Good Thing: Multiple Regression. . . . . . . . 233

Understanding the Basics of Multiple Regression. . . . . . . . . . . . . . . . 234

Defining a few important terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Being aware of how the calculations work . . . . . . . . . . . . . . . . . . . 235

Executing a Multiple Regression Analysis in Software. . . . . . . . . . . . . 236

Preparing categorical variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

Recoding categorical variables as numerical. . . . . . . . . . . . . . . . . . 237

Creating scatter charts before you jump into multiple

regression analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

Taking a few steps with your software. . . . . . . . . . . . . . . . . . . . . . . 241

Interpreting the Output of a Multiple Regression Analysis. . . . . . . . . 241

Examining typical multiple regression output. . . . . . . . . . . . . . . . . 241

Checking out optional output to request. . . . . . . . . . . . . . . . . . . . . 243

Deciding whether your data are suitable for

regression analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Determining how well the model fits the data. . . . . . . . . . . . . . . . 244