[R-sig-ME] subjects within groups and effects of group

[P Greenwood]

Hello

I wanted some advice about handling subjects within groups and effects of group (randomly assigned).  I want to predict reaction time (RT) as a function of  “Condition,”  alpha band power (PzAlpha), and drive. People (subjects) are randomly assigned to Condition, of which there are two.  Each person has data from 5 drives, and for each drive there are 10 trials.  There are 19 subjects in one group and 20 in the other.

My question is this: Am I handling the “between subjects” factor of Condition correctly?  Also, am I treating subjects within group correctly?  I am pasting in some of my data.  The output is below.  

Regards

Pam Greenwood

library(lme4)
library(lmerTest)
INFAST_Behavioral <- read.csv(“….
na.omit(INFAST_Behavioral)
INFAST_Behavioral$RT = scale(INFAST_Behavioral$RT, center = TRUE, scale = TRUE)
INFAST_Behavioral$PzAlpha = scale(INFAST_Behavioral$PzAlpha, center = TRUE, scale = TRUE)
sumModelInteraction <- lmer(RT ~ 1 + (Condition + PzAlpha + Drive + PzAlpha*Condition) + (1 | subject) + (1 | trial), data = INFAST_Behavioral)
summary(sumModelInteraction)

subject	Condition		Drive		trial	FzAlpha	CzAlpha	PzAlpha	FzTheta	CzTheta	PzTheta	FzDelta	CzDelta	PzDelta		RT	ACC
1	HumanLanguage	1	1	-1.41	-4.3585	-5.5431	6.1516	1.5911	3.6247	22.38	18.181	13.812		1568.984857	1
1	HumanLanguage	1	2	-7.8605	2.0156	4.7392	15.992	12.122	6.9088	26.861	20.592	16.326	1721.359714	1
1	HumanLanguage	1	3	-2.6982	-5.6067	-10.038	6.285	5.5172	1.2894	13.565	12.981	11.63	1257.092571	1
1	HumanLanguage	1	4	3.3975	4.8789	-1.3249	7.0177	9.6703	6.1539	10.231	12.261	12.485	1559.461429	1
…(skipping to Subject 2)
2	HumanLanguage			1	1	1.6791	2.8887	0.28174	-11.387	-9.9352	3.5936	-1.5767	3.9401	6.7201		1302.328857	1
2	HumanLanguage	1	2	-13.284	-8.2603	-6.6124	-5.9373	-8.7551	0.10394	4.5621	10.204	12.261	969.0088571	1
2	HumanLanguage	1	3	-0.048973	1.1329	0.67399	-2.1432	2.5077	-2.4641	9.4667	10.883	7.1396	721.3997143	1
2	HumanLanguage	1	4	5.0779	6.8916	6.3892	-1.8682	3.1637	7.9712	8.0994	10.883	10.975	707.1145714	1
2	HumanLanguage	1	5	-7.0495	-2.782	3.1668	8.4332	10.646	9.3726	-3.5937	-7.3769	5.4472	892.8214286	1
2	HumanLanguage	1	6	-1.462	-8.1223	-6.5896	-10.895	-5.6311	0.39941	7.5473	12.783	14.698	611.8802857	1
2	HumanLanguage	1	7	-2.6402	-5.1213	-3.7372	3.4542	4.2234	-0.99898	1.4089	4.1976	0.56587	761.8742857	1
2	HumanLanguage	1	8	3.4393	4.6302	1.5525	1.4604	3.1716	3.1622	-2.3427	2.908	4.2259	680.9251429	1
2	HumanLanguage	1	9	-0.81024	-0.21642	-2.3876	2.5839	4.7307	1.5441	3.3761	8.4485	12.02	769.0168571	1
2	HumanLanguage	1	10	-6.4045	-4.4937	-2.2449	0.94456	2.7048	0.65565	-1.9791	0.26436	1.8435	885.6788571	1

Results:

Linear mixed model fit by REML t-tests use Satterthwaite approximations to degrees of freedom [
lmerMod]
Formula: RT ~ 1 + (Condition + PzAlpha + Drive + PzAlpha * Condition) +  
    (1 | subject) + (1 | trial)
   Data: INFAST_Behavioral

REML criterion at convergence: 3876.4

Scaled residuals: 
    Min      1Q  Median      3Q     Max 
-3.4308 -0.5227 -0.1194  0.3547  8.4095 

Random effects:
 Groups   Name        Variance Std.Dev.
 subject  (Intercept) 0.580073 0.76163 
 trial    (Intercept) 0.004778 0.06912 
 Residual             0.434918 0.65948 
Number of obs: 1839, groups:  subject, 39; trial, 10

Fixed effects:
                                  	 Estimate 		Std. Error         df t value Pr(>|t|)   
(Intercept)                        -0.27054    0.17607   40.80000  -1.537  0.13213   
ConditionMachineLang 0.41644    0.24595   36.90000   1.693  0.09884 . 
PzAlpha                             0.01192    0.02411 1797.40000   0.494  0.62117   
Drive                               0.02948    0.01083 1788.40000   2.722  0.00655 **
ConditionMachineLanguage:PzAlpha   -0.01998    0.03476 1803.10000  -0.575  0.56560   
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
P.M. Greenwood, Ph.D.
Associate Professor of Psychology
Editorial Board, NeuroImage
David King Hall 2052
George Mason University
MSN 3F5, 4400 University Drive
Fairfax, VA 22030-4444

Ph: 703 993-4268
fax: 703 993-1359
email: Pgreenw1 at gmu.edu
http://psychology.gmu.edu/people/pgreenw1


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