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

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

Subject |
RE: st: Number Needed to be Treated (NNT) |

Date |
Wed, 1 Aug 2007 18:06:09 +0100 |

Svend's -syntax- allows -if- and -in-, but the program ignores them. In addition, I suggest adding a -version- statement and some rounding in the display. I have not tested whether this works in Stata 7. * touched by NJC *! 1.0.0 Svend Juul 1 August 2007 program cs_nnt version 8.2 syntax varlist(min=2 max=2 numeric) [if] [in] marksample touse qui count if `touse' if r(N) == 0 error 2000 cs `varlist' if `touse' local nnt = 1/r(rd) local ub = 1/r(ub_rd) local lb = 1/r(lb_rd) display _n as txt " NNT = " /// as res %3.2f `nnt' /// as txt " (`c(level)'% CI: " /// as res %3.2f `ub' /// as txt "; " as res %3.2f `lb' as txt ")" end Nick n.j.cox@durham.ac.uk Svend Juul > Carlo wrote: > > Is there any Stata 9.2 function for calculating the patient > number needed to > be treated (NNT) in order to prevent one case of a given > disease? (Search do > not support this query) > > - and Ronán Conroy followed up: > ... it would make a useful addition to the -epitab- commands > which calculate > absolute risk reduction, together with its associated > confidence interval, > for those who are not brave enough to calculate something by hand. > > ------------------------------------------------------------ > > This calls for a small program. I wrote cs_nnt.ado and put it in > c:\ado\personal. > > program cs_nnt > syntax varlist(min=2 max=2 numeric) [if] [in] > cs `varlist' > local nnt = 1/r(rd) > local ub = 1/r(ub_rd) > local lb = 1/r(lb_rd) > display _newline " NNT = " `nnt' " (`c(level)'% CI: " > `ub' "; " `lb' ")" > end > > It utilizes -cs- (see [ST] epitab); the -return list- command > discloses among > other things: r(rd), r(ub_rd), and r(lb_rd). > > Actually, the -lbw- dataset is from a case-control study, so > don't imagine > it can be used to estimate absolute risks, risk differences, or NNT. > Here we forget this reservation: > > . webuse lbw.dta > (Hosmer & Lemeshow data) > > . cs_nnt low smoke > > | smoked during pregnancy| > | Exposed Unexposed | Total > -----------------+------------------------+------------ > Cases | 30 29 | 59 > Noncases | 44 86 | 130 > -----------------+------------------------+------------ > Total | 74 115 | 189 > | | > Risk | .4054054 .2521739 | .3121693 > | | > | Point estimate | [95% Conf. Interval] > |------------------------+------------------------ > Risk difference | .1532315 | .0160718 > .2903912 > Risk ratio | 1.607642 | 1.057812 > 2.443262 > Attr. frac. ex. | .377971 | .0546528 > .5907112 > Attr. frac. pop | .1921887 | > +------------------------------------------------- > chi2(1) = 4.92 Pr>chi2 = 0.0265 > > NNT = 6.5260736 (95% CI: 3.4436304; 62.220882) > > Interpretation: For each 6.5 smokers we produce one low > birthweight baby. > The confidence interval tells about the uncertainty of this estimate. > > NNT is claimed to make communication with patients easier, > and that may > sometimes be the case. But things may also be made more > difficult. Here > we try with a non-existent association, creating the random binary > variable -x-: > > . set seed 12345 > . gen x = uniform()>0.5 > . cs_nnt low x > > | x | > | Exposed Unexposed | Total > -----------------+------------------------+------------ > Cases | 34 25 | 59 > Noncases | 65 65 | 130 > -----------------+------------------------+------------ > Total | 99 90 | 189 > | | > Risk | .3434343 .2777778 | .3121693 > | | > | Point estimate | [95% Conf. Interval] > |------------------------+------------------------ > Risk difference | .0656566 | -.0659201 > .1972332 > Risk ratio | 1.236364 | .8040256 > 1.901177 > Attr. frac. ex. | .1911765 | -.2437415 > .4740101 > Attr. frac. pop | .1101695 | > +------------------------------------------------- > chi2(1) = 0.95 Pr>chi2 = 0.3306 > > NNT = 15.230769 (95% CI: 5.0701399; -15.169883) > > The confidence interval is complicated; it actually means two > intervals: > 5.07 to infinity > minus infinity to -15.17 > - the negative numbers meaning that exposure to x may prevent low > birthweight. > > I am not sure that -cs_nnt- deserves to be made more public than this, > but those who want can go on playing with it. * * 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/

**Follow-Ups**:**R: st: Number Needed to be Treated (NNT)***From:*"Carlo Lazzaro" <carlo.lazzaro@tin.it>

**References**:**Re: st: Number Needed to be Treated (NNT)***From:*"Svend Juul" <SJ@SOCI.AU.DK>

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