[R] releveling a numeric by factor interaction

Christy Denckla
Tue, 03 Aug 2010 21:36:28 -0000

Can anyone help me with the necessary code to relevel a numeric*factor
interaction term in a linear model?  I would like to report the estimate,
std. error and t-value for the reference factor.

First, I estimated a linear model with dummy variables and was able to
retrieve model estimates for the reference factor using relevel.

for example:

> summary(update(mod.mod, . ~ . - dummy +
+ relevel(dummy, ref="m")))

However, when I attempt to follow the same procedure to estimate the
reference factor interacted with a numeric predictor, I receive an error
message.

> summary(update(hdanxinteract.mod, . ~ . - numeric:dummy +
+ relevel(numeric:dummy, ref="m")))

Error in relevel.default(numeric:dummy, ref = "m") :
  'relevel' only for factors
In addition: Warning messages:
1: In numeric:dummy :
  numerical expression has 152 elements: only the first used
2: In numeric:dummy :
  numerical expression has 152 elements: only the first used

If relevel is only for factors, how can I perform an analogous procedure for
factor by numeric interactions?

Thank you in advance for advice,
Christy
Adelphi University
Doctoral Candidate

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