Output from a SPLINE effect with DETAILS - how to interpret?

[plf515]

I am running SAS 9.4 TS 1M6 on Windows.

In GLIMMIX, I had the following code:

 effect splineaccent = spline(accent/naturalcubic details) ;

the results included:

Knots for Spline Effect splineaccentKnot Number Accent123

25

50

75

which, to me, indicates 3 knots at 25, 50 and 75 but also

Basis Details for Spline Effect splineaccentColumn Power Break Knot123

0
1
3	25

which I am not sure of.  And the solutions for fixed effects showed 3 effects for spline. Are those 0-25, 25-50, 50-75 and an unshown (presumably 0) effect for 75 and up? Or is it the other way round (with the 0 effect for 0-25)? Or something else?

[Rick_SAS · Accepted solution]

My article on "Regression with Cubic Splines" contains many references for splines and the EFFECT statement in SAS, including the doc and a SAS Usage Note.

Usage Note 57975: Understanding splines in the EFFECT statement

Doc on Splines and Spline Basis: 

The three fixed effects are the spline basis functions, which you can output by using the OUTDESIGN= option on the PROC GLMSELECT statement, as shown in my article. Pictures of the spline basis functions are in the doc link. The fixed-effect model is in terms of the linear combinations of the spline basis functions.