[Rd] How to evaluate an lm() object for generating warning statement in a function

From: Michael Rennie <mrennie_at_utm.utoronto.ca>
Date: Tue 23 Jan 2007 - 04:49:04 GMT

Hi, there

I tried this post on R-help but did not generate any replies, so I thought I might try the waters here since it's a more programming-based group.

I have written a function that will allow me to calculate the variance components for a model II (random effects) single factor ANOVA (perhaps this is
me re-inventing the wheel, but I handn't yet come across anything in R that does this for me, but I'm sure people will let me know from this posting if there are). The function takes as it's argument an object which holds the results of an lm call. e.g.,


The function is as follows (some comments included):


                 anovmod<-anova(object.lm) #get the anova table for the lm
                 MStreat<-anovmod[1,3]; MSErr<-anovmod[2,3] #extract Mean 
                 dataframe<- as.data.frame(object.lm[13]) #gets the data 
that went into the lm
                 ni <- tapply(dataframe[,1], dataframe[,2], length) #number 
of cases per treatment level
#required to calculate variance components
                 stot<-s2a + MSErr
                 treatvar<-s2a/stot*100  #calculate variance components as 
a percentage of the total
                 p.var.treat=treatvar, p.var.err=errorvar)


I'd like to include a "warning" statement in the function that returns something like 'function requires arguments that are objects of the form obj<-lm
(y~x)', but I need a case to evaluate the object against in order to throw the warning.

Any advice?

I feel like my best opportunity is after the first line into the function, where I ask for the ANOVA table. Since this is a 2 X 5 table, presumably I should be able to evaluate it against the size of that table? Any thoughts on how to check that? I welcome any suggestions.



Michael Rennie
Ph.D. Candidate, University of Toronto at Mississauga 3359 Mississauga Rd. N.
Mississauga, ON L5L 1C6
Ph: 905-828-5452 Fax: 905-828-3792

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