Hi, I have a nonlinear model with residuals that are negatively autocorrelated at short distances.
I can find no spatial correlation structures in nlme that can easily handle negative autocorrelation as most have bounds on parameter values so that correlation is from 0 to 1. First, is there something I am missing? I tried to roll my own by calculating a correlation matrix with some negative values off the diagonal and then setting the correlation structure as follows within the function gnls: correlation=corSymm(corr9x[lower.tri(corr9x)]) where corr9x is the n x n matrix of correlations that I set up based upon distances between points in the data set. Some of these correlations are positive and some are negative. They are based on a Moran's I correlogram that I calculated from the residuals returned from a gnls model fit with NO spatial correlation employed. I get the following error: Initial values for corSymm do not define a positive-definite correlation structure I am unsure if the matrix is rejected out of hand because it contains negative values or if there is something I can do to coerce it. I have checked the lower triangle matrix returned and it matches what I intended. Any input is appreciated. Seth Myers -- View this message in context: http://r.789695.n4.nabble.com/negative-correlation-with-corSymm-in-nlme-or-another-way-tp4666357.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
