Dear R gurus
I am analysing data from a study of behaviour and shade utilization of
chimpanzees. I am using GLMs in R (version 2.13.0) to test whether shade/sun
utilization is predicted by behaviour observed. I am thus interested in
whether an interaction of behaviour (as a predictor) and presence in the
sun/shade (also predictor) predicts the counts I have for the respective
categories.
I have my data organised as such:
behaviour location specific total Travel Sun 131 303 Travel Shade 172 303
Foraging Sun 248 651 Foraging Shade 403 651 Vigilance Sun 97 224
Vigilance Shade 127 224 Rest Sun 502 1143 Rest Shade 641 1143 Abnormal
Sun 33 58 Abnormal Shade 25 58 Play Sun 58 173 Play Shade 115 173
SelfGrooming Sun 183 595 SelfGrooming Shade 412 595 SocialGrooming Sun 59
358 SocialGrooming Shade 299 358 Other Sun 4 39 Other Shade 35 39 Hidden
Sun 120 656 Hidden Shade 536 656
I have coded the response variable as a specific count of the times
individuals were in the sun or shade, for each behaviour, out of a total
number of times a specific behaviour was observed (regardless of location
[sun/shade]). These are represented by the columns 'specific' and 'total'
respectively. I had originally coded these values as a proportion variable,
but had similar mismatch problems between R and Statistica (as described
below). The GLM I am running is a binomial one (as my count response
variables are divided dichotomously by the sun/shade predictor variable)
with a logit link function. My problem is this: I originally ran the data
through another stats program (Statistica) and got significant effects for
all first- and second-order effects. When I examined the raw data, the
patterns seen in the raw data suggested that these outcomes (of the GLM)
conformed to the raw data (i.e. confirmed the GLM results). I then ran the *
same* data through R using the following code:
> behdata<-read.csv("behaviourshade.csv",header=TRUE)
> behdata #Just to check that everything is there and working...
> behav<-behdata$behaviour
> loc<-behdata$location
> prop<-behdata$proportion
> spec<-behdata$specific
> total<-behdata$total
> model<-glm((cbind(spec,total))~behav*loc,family=binomial,data=behdata)
> summary(model)
Call:
glm(formula = (cbind(spec, total)) ~ behav * loc, family = binomial,
data = behdata)
Deviance Residuals:
[1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -0.8416 0.2393 -3.517 0.000436 ***
behavForagingfeeding 0.3620 0.2475 1.463 0.143585
behavHidden 0.6395 0.2462 2.597 0.009396 **
behavOther 0.7334 0.3338 2.197 0.028044 *
behavPlay 0.4332 0.2678 1.618 0.105739
behavRest 0.2632 0.2443 1.077 0.281320
behavSelfGrooming 0.4740 0.2477 1.914 0.055644 .
behavSocialGrooming 0.6615 0.2518 2.627 0.008602 **
behavTravel 0.2753 0.2576 1.069 0.285142
behavVigilance 0.2741 0.2638 1.039 0.298732
locSun 0.2776 0.3237 0.858 0.391077
behavForagingfeeding:locSun -0.7631 0.3382 -2.257 0.024036 *
behavHidden:locSun -1.7743 0.3436 -5.164 2.41e-07 ***
behavOther:locSun -2.4467 0.6593 -3.711 0.000206 ***
behavPlay:locSun -0.9621 0.3772 -2.551 0.010752 *
behavRest:locSun -0.5221 0.3318 -1.573 0.115615
behavSelfGrooming:locSun -1.0892 0.3406 -3.197 0.001387 **
behavSocialGrooming:locSun -1.9005 0.3615 -5.258 1.46e-07 ***
behavTravel:locSun -0.5499 0.3533 -1.556 0.119597
behavVigilance:locSun -0.5471 0.3632 -1.506 0.131952
---
Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1
(Dispersion parameter for binomial family taken to be 1)
Null deviance: 4.5553e+02 on 19 degrees of freedom
Residual deviance: -1.3700e-13 on 0 degrees of freedom
(19 observations deleted due to missingness)
AIC: 165.12
Number of Fisher Scoring iterations: 3
When I ran it through R I got VERY different results. The R GLM suggested
that behaviour and the behaviour*location interactions were significant
predictors of the counts but the location (sun/shade) was not. In addition,
within each factor, where differences lay were very different between tests.
While I understand that it is entirely possible to have significant 2nd
order interactions when 1st order effects may not be significant, these
patterns described seem to defy apparently obvious patterns in the raw data.
To further complicate things, I was reading through Crawley's R book where
he describes the relationships between orthogonal and non-orthogonal studies
and the order of factor entry; how order can complicate GLM type analyses
for non-orthogonal studies. My data are orthogonal, yet the order in which I
enter variables into the model influences which factors are significant and
which are not. Why is this the case? Am I doing something wrong here with my
data structure or coding? For example, if I switch the order from
'behav*loc' to 'loc*behav' I get yet another set of results that match
neither the first R GLM results, nor the original results from the other
program.
I have checked the model for overdispersion and found that it is not
overdispersed. What am I doing wrong here? How can the same dataset be
generating such vastly different outcomes? I suspect that it may lie in the
way in which the model is fitted (R does iteratively reweighted least
squares whereas, Statistica may use something entirely different; what
exactly, I have no clue...) but I am none the wiser in this regard, so I
really don't know.
Regards, in despiration...
Luke
*PhD Candidate*
*School of Animal, Plant and Environmental Sciences*
*University of the Witwatersrand*
*Johannesburg, South Africa*
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