I'm fitting a mixed multilevel model in R using the nlme package, and I don't understand what is the difference between the covariance structures `corAR1`

and `corCAR1`

. Can someone help me?

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#### Best Answer

Adding an answer because this question was never directly answered and I came here looking for the answer. Looking at the NLME documentation, `corAR1`

represents an autocorrelation structure of order 1. The `corCAR1`

argument represents an autocorrelation structure of order 1, with a continuous time covariate. In addition, Singer and Willet (2003) note the important distinction is that `corCAR1`

allows for unequally spaced time covariates or observations, while corAR1 requires equally spaced time intervals.

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