Page 16 - Intermediate Statistics for Dummies

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Table of Contents
Checking the Model’s Fit (The Data, Not the Clothes!).............................76
Defining the conditions .......................................................................77
Finding and exploring the residuals...................................................78
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Using r to measure model fit..............................................................81
Scoping for outliers..............................................................................82
Making Correct Conclusions.........................................................................84
Avoiding slipping into cause-and-effect mode..................................84
Extrapolation: The ultimate no-no .....................................................85
Knowing the limitations of a simple linear regression model ........86
Chapter 5: When Two Variables Are Better than One:
Multiple Regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87
The Multiple Regression Model....................................................................87
Discovering the uses of multiple regression.....................................88
Looking at the general form of the multiple regression model ......88
Stepping through the analysis............................................................89
Looking at X’s and Y’s....................................................................................89
Collecting the data.........................................................................................90 xiii
Pinpointing Possible Relationships .............................................................92
Making scatterplots .............................................................................92
Correlations: Examining the bond......................................................93
Checking for Multicolinearity.......................................................................95
Finding the Best-Fitting Model......................................................................96
Getting the multiple regression coefficients.....................................97
Interpreting the coefficients ...............................................................98
Testing the coefficients........................................................................99
Predicting Y by Using the X Variables.......................................................101
Checking the Fit of the Model.....................................................................102
Noting the conditions ........................................................................102
Plotting a plan to check the conditions...........................................102
Chapter 6: One Step Forward and Two Steps Back:
Regression Model Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107
Getting a Kick out of Estimating Punt Distance........................................108
Brainstorming variables and collecting data..................................108
Examining scatterplots and correlations ........................................110
Using the Forward Model Selection Procedure........................................113
Adding variables — one at a time ....................................................113
How well does the model fit?............................................................115
Applying forward selection to punt distances ...............................116
Shifting into Reverse: The Backward Model Selection Procedure.........118
Eliminating variables one by one .....................................................119
Assessing model fit ............................................................................120
Kicking variables out to estimate punt distance............................120