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From |
"Steichen" <steichen@triad.rr.com> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: Re: Problems programming nl least squares |

Date |
Mon, 17 Nov 2003 11:34:48 -0500 |

Paola -- This is usually an indication that the initial values for the parameters are poor, leading to a solution where the prediction is flat for some parameters (i.e., no variation). Try different starting values. Tom ----- Original Message ----- From: "Paola De Agostini" <pdeago@essex.ac.uk> To: <statalist@hsphsun2.harvard.edu> Sent: Monday, November 17, 2003 10:41 AM Subject: st: Problems programming nl least squares > Dear statalisters, > > I am having some trouble programming an nl-command. I would like to > program the following function: > > replace `1'= ($B0 + $B1*x1 + $B2*x2 + $B3*x3)*exp( $GF*Z) > > Where X are individual characteristics, Z is ln of hhld income and $Bi > and $GF are parameters to be estimated. > > My problems arise when I increase the number of explanatory vars X. In > this case my command line becomes too long (more than 255 characters). > Therefore I defined the sum of beta times X over `i' using a loop as > shown below. > > program define nlexercise4_1 > version 8.0 > if "`1'"=="?" { > > [parameters definition] > [parameters initialization] > > exit > } > > local sum = 0 > forvalues i = 1/3 { > local sum= `sum' + ${B`i'}* x`i' > } > > replace `1'= ($B0 + `sum')*exp( $GF*Z) > End > > Results: > > (stata4_1) > ------------------------------------------------------------------------ > ------ > milkcq | Coef. Std. Err. t P>|t| [95% Conf. > Interval] > -------------+---------------------------------------------------------- > ------ > B0 | 178.1014 8.751873 20.35 0.000 160.9446 > 195.2581 > B1 | 116.3971 . . . . > . > B2 | 142.4194 . . . . > . > B3 | 114.4872 . . . . > . > GF | .0219544 .0100062 2.19 0.028 .0023387 > .0415701 > ------------------------------------------------------------------------ > ------ > * Parameter B3 taken as constant term in model & ANOVA table > (SEs, P values, CIs, and correlations are asymptotic approximations) > > > So, it looks like Stata does not recognize B1, B2 and B3 as parameters > and does not estimate them. > > Has anybody encountered a similar problem before? Any hints, > suggestions, references would be very much appreciated :-)) > > Thank you, > > Paola De Agostini > > * > * For searches and help try: > * http://www.stata.com/support/faqs/res/findit.html > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Problems programming nl least squares***From:*"Paola De Agostini" <pdeago@essex.ac.uk>

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