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However, a restricted cubic spline may be a better choice than a linear spline when working with a very curved function. When using a restricted cubic spline, one obtains a continuous smooth function that is linear before the ﬁrst knot, a piecewise cubic polynomial between adjacent knots, and linear again after the last knot. Example 3. Even though traditional cubic splines are well behaved for many applications, it does not prevent overshoot at intermediate points. This is illustrated in Figures 1 and 2, where a natural cubic spline is fitted to hypothetical and somewhat unusual distillation and pump curves. Once the dose-relationship between the TSH level and the CKD risk was established, a restricted cubic spline regression model was used to reveal a fixed-effects potential nonlinearity model. Re: How to plot Restricted Cubic Spline in PROC LOGISTIC (BY IMPUTATION) 1. You need to evaluate the final model, which is defined by the parameter estimates table. 2. Because your model is defined in terms of splines , you should output the design matrix, which will contain the spline1-spline3 variables. . Similarly, the natural splines create an inflection point (i.e., the second-derivative is zero at the endpoints). For this particular problem the not-a-knot splines work best near the large values of x. On the Runge phenomenon example from before, cubic spline interpolants perform better than high-degree polynomials as shown in this next figure. A cubic spline hazard model where the tails are linearly constrained (Stone and Koo, 1985) has considerable flexibility in describing data which has been generated from distributions having a. Note that a "cubic" spline is a third order polynomial, which means that it has 4 coefficients. Consequently if there are more than 4 points, we cannot find one that has all points on its curve. So either the spline is actually of -th order, or the spline consists of segments each with its own coefficients, or the spline is a best-fit function. If you really want to use cubic splines, one option would be to use the recently published -xblc- command. To install -xblc- use the following commands: Code: net sj 11-3 st0215_1 net install st0215_1. Here's an example of -xblc- using the cancer dataset that comes with Stata: Code:.