[R] covariate or predictor

Rolf Turner r.turner at auckland.ac.nz
Wed Nov 26 03:13:02 CET 2014

On 26/11/14 13:57, Kristi Glover wrote:
> Hi,
> 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,
> -15L))
> attach(A)
> b<-aov(Strength~Diameter)
> summary(b)
> c<-aov(Strength~Diameter+as.factor(Machine))
> summary(c)
> 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.

(1) Please don't post in HTML; messages with code in them become unreadable.

(2) This isn't really an R question is it?  Possibly better posted to 

(3) What in your mind is the difference between "covariate" and 
"predictor"?  In my (possibly limited) understanding, the words are 

(4) Are you perhaps concerned with the difference between a continuous 
predictor and a categorical (factor) predictor?

(5) In your example "Machine" is pretty clearly *categorical*; the
numbers 1, 2, and 3 are just *labels* for the machines; their numerical 
value is of no significance.  The labels could just as well be "A", "B"
and "C", or "melvin", "irving" and "clyde".

(6) OTOH "Diameter" is pretty obviously interpretable as a *numerical*

(7) I have no idea what you mean by "If it is so, it means that 
co-variate is  dummy variable, right????"  Would you care to translate 
that into English?


Rolf Turner

Rolf Turner
Technical Editor ANZJS

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