use "C:\Documents and Settings\Administrator\\xxx.dta",clear

twoway scatter y x

line y x

nl (y=({k0}+{k1}*(x-{xc})+{k2}*(x-{xc})^2+{k3}*(x-{xc})^3)*(x<{xc})+({k4}*x+{k5}*x^2+{k6}*x^3)*(x>={xc})),initial(k0 3.9 k1 -0.8 k2 -2.0 k3 -0.7 k4 3.8 k5 -3.1 k6 0.6 xc 2.5)
	  
predict p
graph twoway (scatter y x) (line p x)

=================
k0 3.9 k1 -0.8 k2 -2.0 k3 -0.7 k4 3.8 k5 -3.1 k6 0.6 xc 2.5
================
ѧϰ

http://www.ats.ucla.edu/stat/stata/faq/piecewise.htm

use http://www.ats.ucla.edu/stat/stata/faq/talk, clear

nl (talk={a}*age+{b}*age^2+{c}*age^3) if age<14

nl (talk={a1}*age+{b1}*age^2+{c1}*age^3) if age>=14

=======================================
http://www.ats.ucla.edu/stat/stata/faq/nl_optimal_knots.htm

use http://statistics.ats.ucla.edu/stat/stata/faq/nl.dta, clear

nl (y = ({a1} + {b1}*x)*(x < {c}) + ({a1} + {b1}*{c} + {b2}*(x-{c}))*(x >= {c})),  initial(a1 25 b1 -2 c 10 b2 2)
==========================


	   
	   
	   
