[R] Factor moderators in metafor

[John Hodgson]

I'm puzzled by the behaviour of factors in rma models, see example and
comments below. I'm sure there's a simple explanation but can't see it... 

Thanks for any input

John Hodgson


------------------------------------- code/selected output -----------------


library(metafor)

##    Set up data (from Lenters et al  A Meta-analysis of Asbestos and Lung
Cancer...
##    Environmental Health Perspectives • volume 119 | number 11 | November
2011)
 
KL = c(0.02905, 0.06929, -0.1523, 1.6441, 0.1215, 0.3975, 1.0566, 0.1257,
0.2277, 0.06791, 0.08164, 0.2526, 0.07577, 0.03266, 0.1141, 0.1836, 1.8276,
0.4149, 15.4974)
SE = c(0.006633, 0.09335, 0.08909, 0.4297, 0.07858, 0.1753, 0.3679, 0.1837,
0.2172, 0.2775, 0.4201, 0.1976, 0.7688, 0.06507, 0.06239, 0.09061, 0.9509,
0.2181, 7.331)

VL = SE*SE

amph =   c(0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
mix =    c(0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1)
ftype =  c(0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2) 


factor(amph)
factor(ftype)
factor(mix)

##  Fit ftype...
 
> rma(KL,VL,mods=ftype)


Mixed-Effects Model (k = 19; tau^2 estimator: REML)

tau^2 (estimate of residual amount of heterogeneity): 0.0111 (SE = 0.0095)
tau (sqrt of the estimate of residual heterogeneity): 0.1054

Test for Residual Heterogeneity: 
QE(df = 17) = 43.0937, p-val = 0.0005

Test of Moderators (coefficient(s) 2): 
QM(df = 1) = 1.1069, p-val = 0.2928

Model Results:

         estimate      se    zval    pval    ci.lb   ci.ub   
intrcpt    0.0811  0.0606  1.3380  0.1809  -0.0377  0.2000   
mods       0.0473  0.0449  1.0521  0.2928  -0.0408  0.1353   


Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 


##   why only one df for the 2-level factor?
##   in other words, why isn't the above model the same as the following...


> 
> 
> 
> rma(KL,VL,mods=cbind(amph,mix))

Mixed-Effects Model (k = 19; tau^2 estimator: REML)

tau^2 (estimate of residual amount of heterogeneity): 0.0030 (SE = 0.0046)
tau (sqrt of the estimate of residual heterogeneity): 0.0549

Test for Residual Heterogeneity: 
QE(df = 16) = 37.5762, p-val = 0.0017

Test of Moderators (coefficient(s) 2,3): 
QM(df = 2) = 6.9220, p-val = 0.0314

Model Results:

         estimate      se    zval    pval    ci.lb   ci.ub   
intrcpt    0.0380  0.0402  0.9448  0.3447  -0.0408  0.1169   
amph       0.2879  0.1163  2.4754  0.0133   0.0599  0.5158  *
mix        0.0888  0.0625  1.4199  0.1556  -0.0338  0.2114   

---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 

> 




--
View this message in context: http://r.789695.n4.nabble.com/Factor-moderators-in-metafor-tp4639745.html
Sent from the R help mailing list archive at Nabble.com.