Alex Baugh
2007-Jul-09 19:12 UTC
[R] ANOVA: Does a Between-Subjects Factor belong in the Error Term?
I am executing a Repeated Measures Analysis of Variance with 1 DV (LOCOMOTOR RESPONSE), 2 Within-Subjects Factors (AGE, ACOUSTIC CONDITION), and 1 Between-Subjects Factor (SEX). Does anyone know whether the between-subjects factor (SEX) belongs in the Error Term of the aov or not? And if it does belong, where in the Error Term does it go? The 3 possible scenarios are listed below: e.g., 1. Omit Sex from the Error Term:>My.aov = aov(Locomotor.Response~(Age*AcousticCond*Sex) + Error(Subject/(Timepoint*Acx.Cond)), data=locomotor.tab) note: Placing SEX outside the double paretheses of the Error Term has the same statistical outcome effect as omitting it all together from the Error Term (as shown above in #1). 2. Include SEX inside the Error Term (inside Double parentheses):>My.aov = aov(Locomotor.Response~(Age*AcousticCond*Sex) + Error(Subject/(Timepoint*Acx.Cond+Sex)), data=locomotor.tab) 3. Include SEX inside the Error Term (inside Single parentheses):>My.aov = aov(Locomotor.Response~(Age*AcousticCond*Sex) + Error(Subject/(Timepoint*Acx.Cond)+Sex), data=locomotor.tab) note: Placing SEX inside the single parentheses (as shown above in #3) generates no main effect of Sex. Thus, I'm fairly confident that option #3 is incorrect. Scenarios 1,2, and 3 yield different results in the aov summary. Thanks for your help! Alex -- Alexander T Baugh Institute for Neuroscience Univ. of Texas at Austin Austin, TX 78712 512.475.6164 [[alternative HTML version deleted]]
Peter Dalgaard
2007-Jul-09 20:23 UTC
[R] ANOVA: Does a Between-Subjects Factor belong in the Error Term?
Alex Baugh wrote:> I am executing a Repeated Measures Analysis of Variance with 1 DV (LOCOMOTOR > RESPONSE), 2 Within-Subjects Factors (AGE, ACOUSTIC CONDITION), and 1 > Between-Subjects Factor (SEX). > > Does anyone know whether the between-subjects factor (SEX) belongs in the > Error Term of the aov or not? And if it does belong, where in the Error Term > does it go? The 3 possible scenarios are listed below: > > > > e.g., > > 1. Omit Sex from the Error Term: > > >> My.aov = aov(Locomotor.Response~(Age*AcousticCond*Sex) + Error >> > (Subject/(Timepoint*Acx.Cond)), data=locomotor.tab) > > note: Placing SEX outside the double paretheses of the Error Term has the > same statistical outcome effect as omitting it all together from the Error > Term (as shown above in #1). > > > > 2. Include SEX inside the Error Term (inside Double parentheses): > > >> My.aov = aov(Locomotor.Response~(Age*AcousticCond*Sex) + Error >> > (Subject/(Timepoint*Acx.Cond+Sex)), data=locomotor.tab) > > > > 3. Include SEX inside the Error Term (inside Single parentheses): > > > >> My.aov = aov(Locomotor.Response~(Age*AcousticCond*Sex) + Error >> > (Subject/(Timepoint*Acx.Cond)+Sex), data=locomotor.tab) > > note: Placing SEX inside the single parentheses (as shown above in #3) > generates no main effect of Sex. Thus, I'm fairly confident that option #3 > is incorrect. > > > > Scenarios 1,2, and 3 yield different results in the aov summary. > >You don't generally want terms with systematic effects to appear as error terms also, so 3 is wrong. In 2 you basically have a random effect of sex within subject, which is nonsensical since the subjects presumably have only one sex each. This presumably generates an error stratum with 0 DF, which may well be harmless. That leaves 1 as the likely solution. You'll probably do yourself a favour if you learn to expand error terms, a/b == a + a:b, etc.; that's considerably more constructive than trying to think in terms of whether things are inside or outside parentheses.> > Thanks for your help! > > Alex > > > > > >
Christophe Pallier
2007-Jul-10 06:13 UTC
[R] ANOVA: Does a Between-Subjects Factor belong in the Error Term?
On 7/9/07, Alex Baugh <alex.baugh@gmail.com> wrote:> > I am executing a Repeated Measures Analysis of Variance with 1 DV > (LOCOMOTOR > RESPONSE), 2 Within-Subjects Factors (AGE, ACOUSTIC CONDITION), and 1 > Between-Subjects Factor (SEX). > > Does anyone know whether the between-subjects factor (SEX) belongs in the > Error Term of the aov or not?It does not. If you have between-subjects factors A, B and within-subjects factors X, Y, Z, use: aov( dv ~ a*b*x*y*z + Error(subj/(x*y*z)) The subj/(x*y*z) formula includes subj:x subj:y subj:z and all their interactions as error terms. The effect of a within subject factor 'x' is assessed against the error term subj:x -- Christophe Pallier (http://www.pallier.org) [[alternative HTML version deleted]]