[R] Change properties of line summary in interaction.plot

dadrivr dadrivr at gmail.com
Thu Sep 8 00:31:00 CEST 2011

Here's an example:
> id <- c(17,17,17,18,18,18,19,19,19,20,20,20,21,21,21,22,22,22,23,23,23,24,
>         24,24,25,25,25,26,26,26)
> age <- rep(c(30,36,42),10)
> outcome <-
> c(12,17,10,5,5,2,NA,NA,NA,8,6,5,11,13,10,15,11,15,13,NA,9,0,0,0,20,
>              14,16,1,2,2)
> mydata <- as.data.frame(cbind(id,age,outcome))
> interaction.plot(mydata$age, mydata$id, mydata$outcome, fun = mean, legend
> = FALSE, lty = 1, xtick = TRUE, type = "l")
> How can I make the 'mean' summary line red and thicker?

David Winsemius wrote:
> On Sep 7, 2011, at 4:49 PM, dadrivr wrote:
>> David Winsemius wrote:
>>> What "mean summary line"? I count 8 lines and that matches the number
>>> if id's with complete data.
>> Yea, I don't know why there is no mean summary line showing up.  I  
>> requested
>> it in the interaction.plot statement (fun = mean), and I don't get any
>> errors.
> You didn't get any errors and you _did_ get the means. It's just that  
> you only gave it a dataset that had only one value per category. The  
> mean is defined for a single element vector although the sd and var  
> are not. Look more closely at the example on the help page and you  
> will see that it has more than one value per category defined by the  
> two interaction variables

Okay, I checked the example, and I see what you mean.  Is there a way to
compute an average trajectory (i.e., a line that represents the averages of
all the other lines) and make that line red?  Obviously, I'm fairly new to
this in general, so any guidance would be helpful.

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