On Sat, Jan 8, 2011 at 6:25 AM, Rainer Schuermann <rainer.schuerm...@gmx.net> wrote: > It is _from_ a homework but I have the solution already (explicitly got that > done first!) - this was the pasted Latex code (apologies for that, but in > plain text it looks unreadable[1], and I thought everybody here has his / her > favorite Latrex editor open all the time anyway...). I'm just looking, for my own advancement and programming training, for a way of doing that in R - which, from your and Dennis' reply, doesn't seem to exist.
We do ;-) Is this what you are after? It uses uniroot to find a value of 'x' (days) such that the difference between the given and obtained ps is 0 (over a sensible interval). foo <- function(x, p, mean, sd) { mu <- mean * x sigma <- sqrt((sd^2) * x) p - pnorm(4000, mu, sigma, lower.tail = FALSE) } uniroot(foo, interval = c(90, 110), p = .8, mean = 40, sd = 10) Cheers, Josh > > I would _not_ misuse the list for getting homework done easily, I will not > ask "learning statistics" questions here, and I will always try to find the > solution myself before posting something here, I promise! > > Thanks anyway for the simulation advice, > Rainer > > > (4000 - (40*n)) -329 > [1] --------------- = ---- > 1 200 > (10*(n^-)) > 2 > > > > > On Saturday 08 January 2011 14:56:20 you wrote: >> >> On Jan 8, 2011, at 6:56 AM, Rainer Schuermann wrote: >> >> > This is probably embarrassingly basic, but I have spent quite a few >> > hours in Google and RSeek without getting a clue - probably I'm >> > asking the wrong questions... >> > >> > There is this guy who has decided to walk through Australia, a total >> > distance of 4000 km. His daily portion (mean) is 40km with an sd of >> > 10 km. I want to calculate the number of days it takes to arrive >> > with 80, 90, 95, 99% probability. >> > I know how to do this manually, eg. for 95% >> > $\Phi \left( \frac{4000-40n}{10 \sqrt{n}} \right) \leq 0.05$ >> > find the z score... >> > >> > but how would I do this in R? Not qnorm(), but what is it? >> >> Sounds like homework, which is not an encouraged use of the Rhelp >> list. You can either do it in theory or you can simulate it. Here's a >> small step toward a simulation approach. >> >> > cumsum(rnorm(100, mean=40, sd=10)) >> [1] 41.90617 71.09148 120.55569 159.56063 229.73167 >> 255.35290 300.74655 >> snipped >> [92] 3627.25753 3683.24696 3714.11421 3729.41203 3764.54192 >> 3809.15159 3881.71016 >> [99] 3917.16512 3932.00861 >> > cumsum(rnorm(100, mean=40, sd=10)) >> [1] 38.59288 53.82815 111.30052 156.58190 188.15454 >> 207.90584 240.64078 >> snipped >> [92] 3776.25476 3821.90626 3876.64512 3921.16797 3958.83472 >> 3992.33155 4045.96649 >> [99] 4091.66277 4134.45867 >> >> The first realization did not make it in the expected 100 days so >> further efforts should extend the simulation runs to maybe 120 days. >> The second realization had him making it on the 98th day. There is an >> R replicate() function available once you get a function running that >> will return a specific value for an instance. This one might work: >> > min(which(cumsum(rnorm(120, mean=40, sd=10)) >= 4000) ) >> [1] 97 >> >> If you wanted a forum that does not explicitly discourage homework and >> would be a better place to ask theory and probability questions, there >> is CrossValidated: >> http://stats.stackexchange.com/faq >> >> > >> > Thanks in advance, >> > and apologies for the level of question... >> > Rainer >> > >> > ______________________________________________ >> > 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. >> >> David Winsemius, MD >> West Hartford, CT >> >> > > ______________________________________________ > 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. > -- Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.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.