# [R] nlme & special case of corARMA?

From: Reid Landes <rdlandes_at_gmail.com>
Date: Mon, 11 Feb 2008 21:29:33 -0600

Dear All:

I am trying to fit a special case of a 2-banded Toeplitz correlation structure. A 2-banded Toeplitz has ones on the diagonal, a correlation, RHO1, on the first off-diagonal, and a correlation, RHO2, on the second off-diagonal, with zeros on all subsequent off-diagonals. After reading relevant sections in Mixed-Effects Models in S and S-PLUS (Pinheiro & Bates, 2000) and searching on the R-help archives, I've figured out how to get the 2-banded Toeplitz, but not the desired special case.

In the example below, the initial value RHO1 = 0 and RHO2= -0.3. The output matrix below is an example of the special case I'd like to fit -- a 2-banded Toeplitz constraining RHO1=0.

Fitting the 2-banded Toeplitz structure to the ``Orthodont'' example dataset provided in R-Help, we estimate RHO1 also (since the example matrix below contains INITIAL values).

```---------------------Start R-code & output -------------------------------

```

#This intilizes a 2-banded Toeplitz structure  cs1ARMA <- corARMA(value = c(0,-.3), form = ~ 1 | Subject, p = 2, q = 0)  cs1ARMA <- Initialize(cs1ARMA, data = Orthodont)  corMatrix(cs1ARMA)\$M01

[,1] [,2] [,3] [,4]

```[1,]  1.0  0.0 -0.3  0.0
[2,]  0.0  1.0  0.0 -0.3
[3,] -0.3  0.0  1.0  0.0
[4,]  0.0 -0.3  0.0  1.0

> TOEP2 <- gls(distance ~ Sex * I(age - 11), Orthodont,

+                    correlation = corARMA(value = c(0,-.3),
+                    form = ~ 1 | Subject, p = 2, q = 0),
+                   weights = varIdent(form = ~ 1 | age))

```

#-- Selected output follows-----
Correlation Structure: ARMA(2,0)
Formula: ~1 | Subject
Parameter estimate(s):

Phi1 Phi2
0.3269544 0.4897645

• End R-code & output --------------------------------

I cannot figure out how to restrict RHO1 = 0, while allowing estimation of RHO2.

Maybe an answer lies in specifying a different ``position vector'' other than the default: corARMA(..., form = ~ 1 | Subject ...). (See p226 of Pinheiro & Bates, 2000 for explanation of a position vector.) But I'm not totally sure if I understand the position vector and I know I don't know how it works in R.

Then again, there is likely a completely different way to solve this problem.

Any help will be appreciated!

Reid

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