Make Truncated Power Base for Natural Cubic  Spline		[snp7: STB-22]
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   ^spbase^ xvar [[^if^ exp] [^in^ range] ^, gen(^newvar^)^ 
	 [ ^n^knots^(^#^)^ ^kn^ots^(^#,...,#^)^ ]


Description
-----------

^spbase^ creates a truncated power basis for a natural cubic spline in xvar.
It creates one new variable for each interior knot, named newvar1, newvar2,..,
where "newvar" is the name specified by ^gen^. Additionally a macro called
"newvar" is created  containing the name of xvar and the new variables. 

Options
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^gen(^newvar^)^ is not optional. The basis variables are stored in 
"newvar1",..."newvar"nk"", where "nk" is the number of interior knots.

^knots^ specifies the exact location of the interior knots.

^nknots^ specifies the number of interior knots. (Ignored if the locations
	are specified). 

Remarks
-------

The natural cubic spline has knots at the ends of xvar as well at the interior
sites.  It is constrained to be linear beyond the the extreme knots and to be
everywhere twice continuously differentiable.  By default the interior knots
are placed at equally spaced quantiles. The truncated power basis used is not 
particularly computationally stable.

Example
_______

spbase age, gen(age_b)
logistic case $age_b treat
predict fit,xb
replace fit=xb-_b[treat]*treat
gr fit age,s(i) c(l) sort ti("Relative risk associated with age") l1("logit") 



Author
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        Peter Sasieni
        Imperial Cancer Research Fund, London
        FAX 011-44-71-269-3429


Also see
--------

 Manual:  [4] estimate
    STB:  snp7 (STB-22)
On-line:  ^help^ for ^graph^, ^spline^, and ^sp_adj^