[R] covariate or predictor

Kristi Glover kristi.glover at hotmail.com
Wed Nov 26 01:57:44 CET 2014

I am wondering how I can separate whether it is covariate or predictor in the ANOVA analysis. For example
 A<-structure(list(Machine = c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
2L, 3L, 3L, 3L, 3L, 3L), Diameter = c(20L, 25L, 24L, 25L, 32L, 
22L, 28L, 22L, 30L, 28L, 21L, 23L, 26L, 21L, 15L), Strength = c(36L, 
41L, 39L, 42L, 49L, 40L, 48L, 39L, 45L, 44L, 35L, 37L, 42L, 34L, 
32L)), .Names = c("Machine", "Diameter", "Strength"), class = "data.frame", row.names = c(NA, 
I am confused here whether the "Mechine" is covariate or predictor.  How do I know which one is covariate and predictor? 
If Machine is predictor (just like Diameter), how I am supposed to write in the model?
is the equation (below) for this one in the condition that the Machine is predictor? 
c1<-aov(Strength~Diameter+Machine), ?????. If it is so, it means that co-variate is  dummy variable, right????
Your help will really help me to clear the concept. 



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