l53 - Interpreting Interaction Effects; Interaction Effects...

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Interpreting Interaction Effects; Interaction Effects and Centering – Page 1 Interpreting Interaction Effects; Interaction Effects and Centering Models with interaction effects can be a little confusing to understand. The handout provides further discussion of how interaction terms should be interpreted and how centering continuous IVs (i.e. subtracting the mean from each case so the new mean is zero) doesn’t actually change what a model means but can make results more interpretable. Interaction Effects Without Centering. [This handout was originally done with Stata 7, which has different graphics than later versions of Stata]. This problem is modified from Hamilton’s Statistics with Stata 5 and uses data from a survey of undergraduate students collected by Ward and Ault (1990). DRINK is measured on a 33 point scale, where higher values indicate higher levels of drinking. In the sample the mean of Drink is about 19 and the observed scores range between 4 and 33. GPA is the student’s Grade Point Average (higher values indicate better grades). The average gpa is about 2.81. The range of gpa theoretically goes from 0 to 4 but in actuality the lowest gpa in the sample is 1.45. MALE is coded 1 if the student is male, 0 if Female. MALEGPA = MALE * GPA. First, we regress drink on gpa and male. MODEL I: DRINK REGRESSED ON GPA & MALE, WITHOUT CENTERING . use http://www.nd.edu/~rwilliam/stats2/statafiles/drinking.dta, clear (Student survey (Ward 1990)) . regress drink gpa male Source | SS df MS Number of obs = 218 -------------+------------------------------ F( 2, 215) = 18.36 Model | 1437.71088 2 718.855442 Prob > F = 0.0000 Residual | 8416.31205 215 39.1456374 R-squared = 0.1459 -------------+------------------------------ Adj R-squared = 0.1380 Total | 9854.02294 217 45.4102439 Root MSE = 6.2566 ------------------------------------------------------------------------------ drink | Coef. Std. Err. t P>|t| [95% Conf. Interval] -------------+---------------------------------------------------------------- gpa | -3.4529 .9400734 -3.67 0.000 -5.30584 -1.59996 male | 3.535818 .8649733 4.09 0.000 1.830904 5.240731 _cons | 26.91249 2.7702 9.71 0.000 21.45226 32.37272 ------------------------------------------------------------------------------ The model does not allow for the effects of GPA to differ by gender, but it does allow for a difference in the intercepts. Interpreting each of the regression coefficients, * The constant term of 26.9 is the predicted drinking score for a female with a 0 gpa. No woman in the sample actually has a gpa this low. So, you can interpret this as the depths to which a woman would plunge if she was doing that badly. * For both men and women, each one unit increase in gpa results, on average, in a 3.4529 decrease in the drinking scale. That is, those with higher gpas tend to drink less.
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Interpreting Interaction Effects; Interaction Effects and Centering – Page 2 * On average, men score 3.54 points higher on the drinking scale than do women with the same
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This note was uploaded on 02/29/2012 for the course SOC 63993 taught by Professor Richardwilliams during the Spring '11 term at Notre Dame.

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l53 - Interpreting Interaction Effects; Interaction Effects...

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