Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.
| From | Nick Cox <n.j.cox@durham.ac.uk> |
| To | "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu> |
| Subject | RE: st: exponential extrapolation |
| Date | Tue, 1 Mar 2011 17:00:07 +0000 |
The extrapolation here is linear in the logarithms. That being so, it is an extrapolation based on the last known line segment (16, log(1486)) (17, log(1480)) only. No other information is used. Nick n.j.cox@durham.ac.uk Austin Nichols ipianah nic <ipianahnic@gmail.com>: One way to implement the bad idea: clear input genomes csf genomes csf 1 2045 2 1793 3 1715 4 1665 5 1637 6 1613 7 1596 8 1579 9 1566 10 1554 11 1539 12 1527 13 1516 14 1511 15 1497 16 1486 17 1480 end set obs 100 g lc=ln(csf) replace gen=gen[_n-1]+1 if mi(gen) ipolate lc gen, gen(ilc) e g ic=exp(ilc) sc ic c gen On Tue, Mar 1, 2011 at 4:03 AM, Nick Cox <njcoxstata@gmail.com> wrote: > In fact a look at the archives shows that you asked essentially the > same question on 11 February, and unsurprisingly got a very similar > answer from Maarten Buis and myself. > > So, what is going on? Perhaps you need to find someone in your > institution who can talk you through these things. > > Incidentally, a plot of these data suggests to me that exponential > decline is not the right model. There is curvature not taken out by > looking at the data on log scale. Extrapolating the wrong model over > more than 5 times the range of the observation strikes me as a bad > idea, unless there is a compelling biological argument to the > contrary. > > Nick > > On Tue, Mar 1, 2011 at 8:27 AM, Nick Cox <njcoxstata@gmail.com> wrote: >> This looks more as if you have an exponential decline, not an >> exponential distribution. >> >> As with your previous questions, fit an appropriate model using one or >> more of -regress- after transformation, -glm- with log link, or -nl-. >> Then use -predict-. >> >> Nick >> >> On Tue, Mar 1, 2011 at 7:00 AM, ipianah nic <ipianahnic@gmail.com> wrote: >> >>> my data set exhibits an exponential distribution.how can I >>> extrapolate my data set as in the case below where i have genomes and >>> csf..help please * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/