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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

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

Subject |
RE: st: kdensity |

Date |
Mon, 25 Apr 2005 21:07:27 +0100 |

Let's compare like with like. -cumul- (official Stata) produces a cumulative distribution, leaves it in memory as a variable, but does not plot it. -distplot- [sic] (SJ) produces a cumulative distribution on the fly, and does plot it. (It can do more than that, which makes it more useful, but that is a side issue.) The underlying calculations are the same. If we take your example, suppress the -nograph- and insert the -sort- which is needed, then it is clearer what is going on: sysuse auto (1) kdensity mpg, g(a b) cumul b, g(cb) line cb b, sort (2) distplot line mpg What you are doing is comparing (1) the integral of a smoothed density function (2) a unsmoothed cumulative distribution function. (1) is indeed smoother than (2). It would be surprising if it were not. But this is nothing to do with -cumul- and everything to do with what you did with -kdensity-. That said, I prefer to get smoother cumulative distribution functions directly from estimated quantiles. Nick n.j.cox@durham.ac.uk adiallo5@worldbank.org > The cumul command provides a smoother plot than displot. > > e.g.: > > sysuse auto > kdensity mpg, g(a b) nograph > cumul b, g(cb) > line cb b > displot line mpg Nick Cox > If you want a plot of a (?smoothed) distribution function, > this is at best a rather indirect route. > > Note first that -distplot- is a program dedicated > to plotting distribution functions. -search distplot- > points to locations. It is smart enough that you can > go directly to something like > > . distplot line Y X > > without doing overlays. > > If the results are not smooth enough, an alternative is > to base a plot on estimated rather than observed quantiles. > > One command for quantile estimation is -hdquantile- > from SSC. > > Nick > n.j.cox@durham.ac.uk > > Amadou Diallo > > > I used to: > > kdensity X, gen(aa bb) nogr > > cumul bb, g(cum_bb) > > ksm cum_bb bb > > ? > > If you want the density at each point > > you could : > > qui cou > > local n = r(N) > > kdensity X, gen(aa bb) nogr n(`n') > > etc... > > Branko Milanovic > > > When you do kdensity X, STATA charts a kernel density fct > of X's. Now, > > is there a command that would allow me to take the density > > function thus > > generated and chart a cumulative density (or distribution) function? > > Ideally, I would like to do that for both densities, that is > > to go from a overlay graph > > > > twoway (kdensity X) (kdensity Y) > > > > To a similar overlay graph of two cumulative density functions. > > > > Or is the only way to use: > > > > kdensity X, gen(aa bb) > > > > And then generate a cumulative function of aa? By the way, I > > tried that but the graph did not turn out well. * * 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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