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From |
"Scott Merryman" <smerryman@kc.rr.com> |

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

Subject |
st: RE: predictnl and nonlinear regression |

Date |
Fri, 5 Aug 2005 21:15:31 -0500 |

> -----Original Message----- > From: owner-statalist@hsphsun2.harvard.edu [mailto:owner- > statalist@hsphsun2.harvard.edu] On Behalf Of Wallace, John > Sent: Friday, August 05, 2005 7:03 PM > To: statalist@hsphsun2.harvard.edu > Subject: st: predictnl and nonlinear regression > > Dear listmembers: > I'm working my way through learning how to use regressions other than OLS > within stata. One model I frequently use in Excel is the exponential fit. > I believe the following is the way to specify that model within Stata's nl > function: > nl(y = {beta}*exp({alpha}*x) > I have used the predict function following this estimation to generate a > fit and graphed the results, which look like the chart I was expecting to > see. I then looked for a way to generate confidence intervals for the > fit, and have run aground. It looks like the predictnl function would do > what I want, but I can't get it to work. Here's what I've tried: > (after first -drop-ing the fit variable I made with -predict-) > > . predictnl fit = predict(fit), ci(ub lb) > Result: fit not allowed \\\ predict(fit) invalid > . predictnl fit = nl (y = {beta}*exp({alpha}*x)), ci(ub lb) > Result: Unknown function nl() > . predictnl fit = y = {beta}*exp({alpha}*x), ci(ub lb) > Result: =exp not allowed You don't need the y term in the -predictnl- statement. For example: . sysuse auto,clear (1978 Automobile Data) . qui nl (price = {beta}*exp({alpha}*mpg)) . predictnl fit = _b[beta]*exp(_b[alpha]*mpg), ci(lb ub) note: Confidence intervals calculated using t(72) critical values. > Also, is it possible to use the stored estimation command interactively to > solve for a specific x? > e.g.: . predicti(0.5) > which returns some value y according to the model? > How about using -nlcom-. For example, if price = 1000: . nlcom (ln(1000/_b[beta]))/_b[alpha] _nl_1: (ln(1000/_b[beta]))/_b[alpha] ---------------------------------------------------------------------------- -- price | Coef. Std. Err. t P>|t| [95% Conf. Interval] -------------+-------------------------------------------------------------- -- _nl_1 | 52.22725 6.330777 8.25 0.000 39.60708 64.84742 ---------------------------------------------------------------------------- -- Hope this helps, Scott * * 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: predictnl and nonlinear regression***From:*"Wallace, John" <John_Wallace@affymetrix.com>

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