We don't do people's homework for them.
But since you seem to have put in at least a little bit of your own effort ..... It is perfectly possible for there to be an interaction without there being main effects. Consider two factors A and B each with two levels. Let mu_11 be the population mean when A is at level 1 and B is at level 1, and so on. Suppose mu_11 = 1, mu_12 = -1, mu_21 = -1, and mu_22 = 1. Then there are no main effects; A averages to 0, as does B. But there is an elephant-ful of interaction. cheers, Rolf Turner cheers, Rolf Turner On 01/13/2013 10:56 AM, theundergrad wrote:
Hi, I am trying to interpret the coefficients in the model: RateOfMotorPlay ~ TestNumber + Sex + TestNumber * Sex where there are thee different tests and Sex is (obviously) binary. My results are: Residuals: Min 1Q Median 3Q Max -86.90 -26.28 -7.68 22.52 123.74 Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) 29.430 6.248 4.710 4.80e-06 *** TestNumber2 56.231 8.837 6.364 1.47e-09 *** TestNumber3 75.972 10.061 7.551 1.82e-12 *** SexM 7.101 9.845 0.721 0.472 TestNumber2:SexM -16.483 13.854 -1.190 0.236 TestNumber3:SexM -24.571 15.343 -1.601 0.111 --- Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 Residual standard error: 40.97 on 188 degrees of freedom Multiple R-squared: 0.3288, Adjusted R-squared: 0.3109 F-statistic: 18.42 on 5 and 188 DF, p-value: 7.231e-15 I am looking for one number that will represent the significance of the interaction term. I was thinking of doing an F test comparing this model to one without the interaction. When I do this, I get a highly significant result. I am not exactly sure how to interpret this. In particular, it seems strange to me to have a significant interaction term without both independent variables being significant. Any advice would be highly appreciated.
______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.