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From |
sjsamuels@gmail.com |

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statalist@hsphsun2.harvard.edu |

Subject |
Re: st: RE: Why don't my IRs and Cox HRs echo each other? |

Date |
Thu, 7 May 2009 12:14:06 -0400 |

-- Chelsea- I would not expect the two kinds of estimates to align. The ratio of Deaths1/PY1 to Deaths2/PY2 is guaranteed to estimate the hazard ratio only in special circumstances. One is that the data are generated bya two-group exponential model with no time-dependent covariates and constant hazards. There could be many reasons for the dissimilar estimates. Kieran has described an important one. A related issue with time dependent covariates is that the risk sets and exposure for different levels of time-dependent covariates may not cover the same time-periods, whereas the Cox model automatically compares people at risk at the same follow-up times. Confounding could also contribute to the discrepancy, even in the ideal exponential case, because you are computing the PY-based hazard ratios one factor at a time. You can explore distributional shapes for time-dependent groups if you designate the groups as strata. There are also some good ideas for plotting in Applied Longitudinal Data Analysis: Modeling Change and Event Occurrence by Judith D. Singer and John B. Willett -Steve On Wed, May 6, 2009 at 7:35 PM, Polis, Chelsea B. <cpolis@jhsph.edu> wrote: > I apologize for the formatting of my table, it looked ok when I sent it. Please let me try again. > > > Variable Deaths PY at risk IR HR 95% CI p-value > > HC use 0.07 > No 91 1262.7 7.21 1.00 > Yes 13 293.0 4.44 0.58 0.32-1.04 > Current age 0.38 > 15-24 20 394.0 5.08 1.00 > 25-34 49 711.8 6.88 0.73 0.43-1.24 > 35+ 35 449.9 7.78 0.68 0.38-1.20 > Sex partners in past year 0.01 > None 18 241.2 7.46 1.00 > One 76 1204.6 6.31 1.31 0.78-2.21 > Two+ 10 109.9 9.10 3.40 1.54-7.54 > > > Dear statalisters, > > I am doing survival analysis on time to death with time-varying covariates on an open, population > -based cohort study. The base sample is essentially a census of individuals in 56 villages, and > I am utilizing information from all female incident HIV seroconverters. > > I computed incidence rates based on the raw data ((number of deaths/person-time at risk)*100 - I obtained > time at risk using the stdes command), but the IRs don't seem to echo trends in the univariate Cox HRs. > > In the sample data below, things appear reasonable for HC use (deaths per 100 person years is lower > if HC=yes, and the HR reflects this). But for current age, deaths are higher in the 25-34 category > than in the 15-24 category, but the HR trends suggests that being 25-34 is protective (though not > significantly). Also, the magnitude seems off, for example, in the variable "Sex partners in past > year" - having two or more seems to more than triple the hazard in the Cox regression, but merely > increases from 7.46 to 9.10 in the deaths per 100 p-y. > > Am I missing something in expecting these numbers to echo trends in each other? Is this just a > matter of non-significance within individual categories? Or a difference in time-to-event versus > person-time analysis? Or because I am doing an analysis with time-varying covariates? Should I not > expect these to align? Any help is appreciated! > * * 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: Why don't my IRs and Cox HRs echo each other?***From:*"Polis, Chelsea B." <cpolis@jhsph.edu>

**st: RE: Why don't my IRs and Cox HRs echo each other?***From:*"Polis, Chelsea B." <cpolis@jhsph.edu>

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