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From |
"Daniel Waxman" <dan@amplecat.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
RE: st: reverse prediction - confidence interval for x at given y in nonlinear model |

Date |
Fri, 26 Oct 2007 11:50:00 -0700 |

Nick, Unfortunately I am away from a computer with Stata installed and have been tapping this out on a cell phone! if dose=0 then the logistic equation reduces to b[zdose] + b[cons] If dose=1, it reduces to just b[cons]. Agreed. Units are irrelevant. I'll tell you what: When I get home on 11/1, i'll start a new thread with a more convincing example (unless somebody wants to agree with me before then)... Dan -----Original Message----- From: "Nick Cox" <n.j.cox@durham.ac.uk> Subj: RE: st: reverse prediction - confidence interval for x at given y in nonlinear model Date: Fri Oct 26, 2007 11:23 am Size: 2K To: <statalist@hsphsun2.harvard.edu> We have been at cross purposes. Your log(0) as 0 is, I see, only a trick that keeps the zeros in the frame _so long as you also have a dummy for 0_. My point is that it does not help in problems in which there is no dummy for 0. Suppose your doses are 0 0.1 0.2 0.3 0.4 0.5 in whatever units are being used. Neither Stata nor the mathematics knows about the units. Then the recipe cond(dose == 0, 0, log(dose)) is not even monotonic in dose. 0 is treated as if were larger than any other positive value in the data! Also if any dose is 1, this recipe treats 0 and 1 as equivalent. Nick n.j.cox@durham.ac.uk * * 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/

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