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Natural spline (i.e. ns() ) with glmer()/lmer()

Hedyeh Ahmadi 
Hello All,

I would like to double check my understanding with the followings:

1 - In a linear mixed model if I use ns() for a continuous variable with for example one knot then I'll have two pieces of natural splines (i.e. third degree polynomial) one before the knot and one after the knot.

2 - But if I run the same ns() in a linear mixed model without any knots, does that mean I am fitting one 3rd degree polynomial between the boundaries (i.e. over the range of my continuous variable)?

Best,

Hedyeh Ahmadi, Ph.D.
Applied Statistician
Keck School of Medicine
Department of Preventive Medicine
University of Southern California

Postdoctoral Scholar
Institute for Interdisciplinary Salivary Bioscience Research (IISBR)
University of California, Irvine

LinkedIn
www.linkedin.com/in/hedyeh-ahmadi<http://www.linkedin.com/in/hedyeh-ahmadi>

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Hedyeh Ahmadi Natural spline (i.e. ns() ) with glmer()/lmer() May 21 Hedyeh Ahmadi Natural spline (i.e. ns() ) with glmer()/lmer() May 21