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\title{X-ray pixel intensities over time}
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The \code{Pixel} data frame has 102 rows and 4 columns of data on the
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pixel intensities of CT scans of dogs over time
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This data frame contains the following columns:
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a factor with levels \code{1} to \code{10} designating the dog
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on which the scan was made
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a factor with levels \code{L} and \code{R} designating the side
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of the dog being scanned
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a numeric vector giving the day post injection of the contrast
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on which the scan was made
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a numeric vector of pixel intensities
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Pinheiro, J. C. and Bates, D. M. (2000) \emph{Mixed-effects Models in
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S and S-PLUS}, Springer.
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Pixel$DS = getGroups(Pixel, ~ 1 | Dog/Side, level = 2)
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fm1 <- lme(pixel ~ day + I(day^2), data = Pixel,
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random = list(DS = ~ 1, Dog = ~ 1))