# [R] A question on dummy variable

David Winsemius dwinsemius at comcast.net
Tue Jan 11 22:34:21 CET 2011

```You are not offering example of real codings but are rather showing
something1 that you think looks like something2 (in R) that looks like
something3 (in a textbook?). My guess is that the something2 might be
contrast matrices or model matrices.

If you want a contrast matrix whose columns sum to zero (which is one
possible situation that some people might call "centered" then look at
the documentation for sum and poly contrasts. If you want to see a
situation where model matrices are constructed which compare to the
overall mean (another possible interpretation of "centered"), then
look at the documentation for model.matrix and run the examples on
that page with a -1 in the formula.

--
David.
On Jan 11, 2011, at 3:18 PM, Christofer Bogaso wrote:

> Dear all, I would like to ask one question related to statistics, for
> specifically on defining dummy variables. As of now, I have come
> across 3
> different kind of dummy variables (assuming I am working with Seasonal
> dummy, and number of season is 4):
>
>> dummy1 <- diag(4)
>> for(i in 1:3) dummy1 <- rbind(dummy1, diag(4))
>> dummy1 <- dummy1[,-4]
>>
>> dummy2 <- dummy1
>> dummy2[dummy2 == 0] = -1/(4-1)
>>
>> dummy3 <- dummy1 - 1/4
>>
>     [,1] [,2] [,3]
> [1,]    1    0    0
> [2,]    0    1    0
> [3,]    0    0    1
> [4,]    0    0    0
> [5,]    1    0    0
> [6,]    0    1    0
>           [,1]       [,2]       [,3]
> [1,]  1.0000000 -0.3333333 -0.3333333
> [2,] -0.3333333  1.0000000 -0.3333333
> [3,] -0.3333333 -0.3333333  1.0000000
> [4,] -0.3333333 -0.3333333 -0.3333333
> [5,]  1.0000000 -0.3333333 -0.3333333
> [6,] -0.3333333  1.0000000 -0.3333333
>      [,1]  [,2]  [,3]
> [1,]  0.75 -0.25 -0.25
> [2,] -0.25  0.75 -0.25
> [3,] -0.25 -0.25  0.75
> [4,] -0.25 -0.25 -0.25
> [5,]  0.75 -0.25 -0.25
> [6,] -0.25  0.75 -0.25
> Now I want to know which type of dummy definition is called Centered
> dummy
> and why it is called so? Is it equivalent to use any of the above
> definitions (atleast 2nd and 3rd?) It would really be very helpful if
> somebody point any suggestion and clarification.
>
> Thanks and regards,
>
> 	[[alternative HTML version deleted]]
>
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