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From |
Austin Nichols <austinnichols@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: making quantiles based on controls |

Date |
Fri, 15 Oct 2010 00:02:31 -0400 |

Delahanty, Ryan <ryan.delahanty@vanderbilt.edu>: First, I don't think that code does anything like what you want; try: webuse nhanes2, clear g bmi=weight/height^2*10000 su bmi, d generate bmiquart = 0 replace bmiquart=1 if 21<bmi<=23 replace bmiquart=2 if 23<bmi<=26 replace bmiquart=3 if 26<bmi ta bmiquart, sum(bmi) Next, anyone with a missing bmi has a bmi "higher" than any finite value, so an open-ended comparison like the last is generally a bad idea. Lastly, and more constructively, you might try a variant on: webuse nhanes2, clear g bmi=weight/height^2*10000 _pctile bmi if race==1, nq(100) g qw20=(bmi<=r(r20)) if bmi<. loc j 20 foreach i in 40 60 80 100 { g qw`i'=(bmi>r(r`j') & bmi<=r(r`i')) if bmi<. loc j=`i' } g qw=qw20+2*qw40+3*qw60+4*qw80+5*qw100 ta qw race, col or webuse nhanes2, clear g bmi=weight/height^2*10000 _pctile bmi if race==1, nq(5) forv i=1/5 { g qw`i'=(bmi<=r(r`i')) if bmi<. } egen qw=rowtotal(qw?) replace qw=6-qw ta qw race, col which could be turned into a -program- with a modicum of effort (and can also be extended to allow svy weights easily). On Thu, Oct 14, 2010 at 11:46 AM, Delahanty, Ryan <ryan.delahanty@vanderbilt.edu> wrote: > I spend way too much time turning things into tertiles, quartiles, etc. > > So say I have to split a continuous variable like bmi into quartiles based on CONTROL values, I have to do this: > > (In this example affection status of 1 = case, 0 = control). > > by affection, sort: summarize bmi, detail > (which tells me the splits, and then I use them like so) > generate bmiquart = 0 > replace bmiquart=1 if 21<bmi<=23 > replace bmiquart=2 if 23<bmi<=26 > replace bmiquart=3 if 26<bmi > > and then I generate the dummies for bmiquart as I do the regression with something like this: > xi, prefix(): logistic affecttion age income education i.bmiquart > > But the above is kind of cumbersome when splitting data a billion different ways. I'd like to substitute the first five lines and instead use a command like this: > xtile bmiquart = bmi, nquantiles(4) > > The problem with this is that this will just split it based on bmi for cases and controls. Is there a simple command or switch that will allow the quantiles to be generated based on the distribution in controls rather than both? > > Thanks, > Ryan * * 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/

**References**:**st: making quantiles based on controls***From:*"Delahanty, Ryan" <ryan.delahanty@vanderbilt.edu>

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