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From |
"Svend Juul" <SJ@SOCI.AU.DK> |

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

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

Date |
Wed, 1 Aug 2007 14:55:12 +0200 |

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. Svend __________________________________________ Svend Juul Institut for Folkesundhed, Afdeling for Epidemiologi (Institute of Public Health, Department of Epidemiology) Vennelyst Boulevard 6 DK-8000 Aarhus C, Denmark Phone: +45 8942 6090 Home: +45 8693 7796 Email: sj@soci.au.dk __________________________________________ * * 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**:**Re: st: Number Needed to be Treated (NNT)***From:*Ronan Conroy <rconroy@rcsi.ie>

**RE: st: Number Needed to be Treated (NNT)***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

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

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