On Nov 29, 2011, at 23:19 , Ben Bolker wrote: > rch4 <rch4 <at> geneseo.edu> writes: > >> >> We need help.... >> >> We are doing a project for a statistical class in and we are looking at >> world record times in different running events over time. We are trying to >> fit the data with a negative exponential but we just cant seem to get a >> function that works properly. >> we have on our x-axis the date and on the y-axis the time(in seconds). So as >> you can imagine, the times have decreased and appear to be approaching a >> limit. Any ideas for a nls function that would work for us would be greatly >> appreciated. >> >> Rob > > I disagree with the other solutions posted here: think you're looking > not for a distribution, but for the change over time.
Not that this is anywhere near my areas of expertise, but wouldn't you want to be even more careful than that? I mean, surely the record time is nondecreasing, and one would expect that the time between records to carry information about the issue (e.g., in a stable situation, it should increase as a lower limit is being approached)? > > You could start with > > fit1 <- lm(log(time)~I(date-date[1])) > > where the intercept will be the *log* of the intercept (value on > the first date) and the slope will be the exponential coefficient. > If you need to be more careful about your statistical assumptions > (e.g. if the variance appears to be homogeneous on the original > scale but not on the log scale) then something like > > fit2 <- nls(exp(logint)*exp(-r*(date-date[1])), > start=...) > > should work. You need to set the starting values appropriately -- the values > from the linear fit above should be pretty good. > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. -- Peter Dalgaard, Professor Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
