# corCompSymm

##### Compound Symmetry Correlation Structure

This function is a constructor for the `corCompSymm`

class,
representing a compound symmetry structure corresponding to uniform
correlation. Objects created using this constructor must later be
initialized using the appropriate `Initialize`

method.

- Keywords
- models

##### Usage

`corCompSymm(value, form, fixed)`

##### Arguments

- value
the correlation between any two correlated observations. Defaults to 0.

- form
a one sided formula of the form

`~ t`

, or`~ t | g`

, specifying a time covariate`t`

and, optionally, a grouping factor`g`

. When a grouping factor is present in`form`

, the correlation structure is assumed to apply only to observations within the same grouping level; observations with different grouping levels are assumed to be uncorrelated. Defaults to`~ 1`

, which corresponds to using the order of the observations in the data as a covariate, and no groups.- fixed
an optional logical value indicating whether the coefficients should be allowed to vary in the optimization, or kept fixed at their initial value. Defaults to

`FALSE`

, in which case the coefficients are allowed to vary.

##### Value

an object of class `corCompSymm`

, representing a compound
symmetry correlation structure.

##### References

Milliken, G. A. and Johnson, D. E. (1992) "Analysis of Messy Data, Volume I: Designed Experiments", Van Nostrand Reinhold.

Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer, esp. pp. 233-234.

##### See Also

##### Examples

```
# NOT RUN {
## covariate is observation order and grouping factor is Subject
cs1 <- corCompSymm(0.5, form = ~ 1 | Subject)
# Pinheiro and Bates, pp. 222-225
fm1BW.lme <- lme(weight ~ Time * Diet, BodyWeight,
random = ~ Time)
# p. 223
fm2BW.lme <- update(fm1BW.lme, weights = varPower())
# p. 225
cs1CompSymm <- corCompSymm(value = 0.3, form = ~ 1 | Subject)
cs2CompSymm <- corCompSymm(value = 0.3, form = ~ age | Subject)
cs1CompSymm <- Initialize(cs1CompSymm, data = Orthodont)
corMatrix(cs1CompSymm)
# }
```

*Documentation reproduced from package nlme, version 3.1-145, License: GPL (>= 2) | file LICENCE*