[R] error for ttest

[1Rnwb]

Hello all,

I have arranged my data as per Dennis's suggestion in this post
http://www.mail-archive.com/r-help@r-project.org/msg107156.html.
the posted code works fine but when I try to apply it to my data, i get ">
u2 <- ddply(xxm, .(plateid, cytokine), as.data.frame.function(f))
Error in t.test.formula(conc ~ Self_T1D, data = df, na.rm = T) : 
  grouping factor must have exactly 2 levels".
Self_T1D has two levels "N" and "Y"

I have used the ddply function to do the mean and sd for the same dataframe
without any issues.
I would appreciate help to solve this.
Thanks
Sharad

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