20 lines
642 B
R
20 lines
642 B
R
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library(survival)
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library(splines)
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# the nsk function should give the same solution as ns, but with a different
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# parameterization
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#
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xx <- runif(500, 1, 100)
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yy <- 10*log(xx) + rnorm(500, 0, 2)
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tdata <- data.frame(xx=xx, yy=yy)
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fit1 <- lm(yy ~ ns(xx, df=4), tdata, model=TRUE)
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fit2 <- lm(yy ~ nsk(xx, df=4, b=0), tdata)
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all.equal(predict(fit1), predict(fit2)) # same solution
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xattr <- attributes(fit1$model[[2]])
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allknots <- sort(c(xattr$knots, xattr$Boundary.knots)) # knots that were used
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pred.knot <- predict(fit1, newdata=list(xx=allknots))
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all.equal(pred.knot[-1] - pred.knot[1], coef(fit2)[-1],
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check.attributes = FALSE)
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