Hi Mario,
On Fri, Jan 28, 2011 at 12:27 PM, gaiarrido <gaiarrido at usal.es>
wrote:>
> Hi,
> I'm trying to make a model in order to know wich factors got?s
influence in
> the intensity of a infection, but just in the individuals who's got
this
> infection. In my data I've got a variable called "prevalence"
with 2 levels:
> 1.- Infected individual
> 0.- Non infected
>
> So what i'm trying to do is a subset in a model like this,
>
model<-aov(intensity~ageandsex*month*zone*year,subset=(prevalence=="1"))
> Is a correct way?
That should work, assuming you have attached the dataframe containing
the variables in your model. A safer way is to use the data agrument
to aov instead:
model<-aov(intensity~ageandsex*month*zone*year, data=X,
subset=(prevalence=="1"))
> I prefer to make the subsets with [], but doesn't work, why
>
>>
model<-aov(hemogregarinas~edadysexo*mes*zona*ano,[prevalencia=="1"])
> Error: inesperado '[' en
> "model<-aov(hemogregarinas~edadysexo*mes*zona*ano,["
>
Well what is [prevalencia=="1"] indexing? Nothing, which isn't
going
to work. You can use
model<-aov(hemogregarinas~edadysexo*mes*zona*ano,
data=X[prevalencia=="1" , ])
> And finally, look at this, i don't know why I get different results
>> summary(hemogregarinas[prevalencia=="1"])
> ? Min. 1st Qu. ?Median ? ?Mean 3rd Qu. ? ?Max.
> ? 1.00 ? ?5.00 ? ?9.00 ? 15.94 ? 19.00 ?173.00
>> summary(hemogregarinas,subset=(prevalencia=="1"))
> ? Min. 1st Qu. ?Median ? ?Mean 3rd Qu. ? ?Max.
> ? 0.00 ? ?4.00 ? ?9.00 ? 14.92 ? 18.00 ?173.00
>
Well I'm not sure, but I suspect it has something to do with attaching
things, and getting confused about where the data is actually coming
from. This gives the same results:
dat <- data.frame(x=c(1,2,2,1,3,4,2,5,3,20), y c(2,1,3,3,2,5,6,4,5,20),
z=c(rep(1, 9),2))
attach(dat)
summary(x[z==1])
summary(subset(x, z==1))
so again, not sure why you're getting something different. Try storing
your data in a data.frame, and use the data arguments instead of
attaching it, and see if you get more sensible results.
Best,
Ista
> Thanks very much
>
> -----
> Mario Garrido Escudero
> PhD student
> Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca.
> Agr?cola
> Universidad de Salamanca
> --
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>
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--
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org