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Don´t know what test i have to use

6 messages · Mario Garrido, David Winsemius, Joshua Wiley +1 more

Mario Garrido
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Hello,
I?m starting with my PhD and I have to stop because i got a little knowledge
in R and statistics.
I?ve got a model of this kind: 
binary response variable: prevalence of infection (0/1)
3 categorical independent variables: sex, month and name of the area  

I was trying with a full model like this, before the simplification

model<-aov(prevalencia~sex*month*area)

but the Fligner test told that i haven?t got homoscedascity, so I suppose I
should trying with glm, with a model

model2<-glm(prevalencia~edad*sexo*mes*zona,binomial)

is that correct? where I must put the link (logit) ?

Thnks very much
David Winsemius
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On Jan 12, 2011, at 12:51 PM, gaiarrido wrote:

            

      
      
      
    

        
Why not read the help page regarding binomial that is on the help page  
for glm. There you will learn what the default link is for binomial.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Joshua Wiley
    

    

      
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Hi,

That is basically correct.  You can specify the link as logit (see my
example), but that is the default so you do not strictly need to in
this case.  II would encourage you to keep your variables
(prevalencia, edad, sexo, mes) stored in a data frame, in which case
you would add the data = argument to glm().

model2 <- glm(prevalencia ~ edad * sexo * mes * zona,
  family = binomial(link = "logit"),
  data = your_dataframe)

Also, you might take a look at ?predict.glm  it has some examples with
binomial data based off the wonderful book by Drs. Venables and
Ripley.  Oh, and finally, if you have 12 levels of months, ? levels of
zones, and 2 levels of sex, you might not want the 4way interactions
that you will get by default from using the '*' operator inside a
formula.  Unless you have a theory that there is an additional effect
of being a middle aged female in the month of July for zone 8, but
not....

Cheers,

Josh

        
On Wed, Jan 12, 2011 at 9:51 AM, gaiarrido <gaiarrido at usal.es> wrote:

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Mario Garrido
    

    

      
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Thanks very much both.
I?m starting playing with it, i was a little afaid because it was part of my
job, but now i've found it very funny.
Josh, I've got just data for 3 representatives months, and it's not a priori
rejectable that could be differences in  the ratio of changes along the
months between the 2 sexes.

Thanks again

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Bert Gunter
    

    

      
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... But I would think that month should be treated as a cyclical
quantity, not as a factor with 12 independent levels, e.g. by
transforming month to  sin( 2*pi*monthNumber/12) .  This assumes 1
year periodicity, which might not be right, of course. Time series
methods could obviously be relevant here. Given the possible
importance of such periodicity and the relative complexity of the
methodology necessary to deal with it properly, you might benefit by
consulting your local statistician for help.

-- Bert

        
On Wed, Jan 12, 2011 at 10:43 AM, Joshua Wiley <jwiley.psych at gmail.com> wrote:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Mario Garrido
    

    

      
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Thanks for the advice, I will use next years. Till know i?ve just got data
for 3 independent months and one of the months it?s the joining for all the
summer because of the small sample size, so, I suppose, I can't use it in
the way you say.

-----
Mario Garrido Escudero
PhD student
Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca.
Agr?cola
Universidad de Salamanca