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From |
"Carlo Lazzaro" <carlo.lazzaro@tin.it> |

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

Subject |
st: R: Choice of analysis time in survival analysis where the hazard is associated with age |

Date |
Mon, 14 Dec 2009 15:32:27 +0100 |

Roland wrote: <Knowing that the hazard is age-dependent I think this may give biased results>. Roland might be interested in exploring the relationship between age and relative hazard of appendicitis by plugging a quadratic term (ie, age^2) into the RHS of Cox regression and then invoke -stcox-. Given: a=coefficient for age; b=coefficient for age^2; x=age the quadratic function: y=ax^2+bx shows a maximum (or minimum) at y'=0 (ie, -b/2a). Provided that the maximum (or minimum) lies within the range of age data, Roland can find out a turning point at which the function changes direction. Depending upon being a maximum or a minimum, up until the turning point the relative hazard increases and then decreases or the other way round. A well detailed example of this topic is reported in: Cleves MA, Gould WW, Gutierrez RG. AnIntroduction to Survival Analysis usung Stata. Revised edition. College Station: Stata Press, 2004: 160-62. HTH and Kind Regards, Carlo -----Messaggio originale----- Da: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di roland andersson Inviato: lunedì 14 dicembre 2009 13.39 A: statalist@hsphsun2.harvard.edu Cc: Terlinder John Oggetto: st: Choice of analysis time in survival analysis where the hazard is associated with age I have a large national dataset and plan to analyse socioeconomic differences in risk of having appendicitis. Persons born after 1949 to end of 1990 and alive at 1/1/1991 (n=4.500.000) are included in the dataset. The dataset contains the date when a person was operated for appendicitis between 1/11/1990 and 31/12/2003 and date of death. The hazard of having appendicitis is agedependent and is increasing till age 13 and then decreasing. Previous collaborators have made Coxregression with time from 1/11/1990 to the appendicitisdiagnosis or censoring as analysistime, and have entered age at operation as covariates to control for the age-dependent differencies in hazard. Knowing that the hazard is age-dependent I think this may give biased results. I therefore wonder if it would be better to use age as analysis time (with late entries) instead of time from start of follow up with adjustment for age. Am I right to think that this will be a better way of controling for the age-dependent differences in hazard? If that is the case may I also include age as a covariate in the analysis or would that complicate things? I have also considered to use a stratified analysis with ageintervals as stratification variable. Can you give some advice on how these ageintervals should be chosen in view of the agedependent increasing and decreasing hazard? I would appreciate your comments. Roland Andersson, MD PhD * * 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/ * * 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: Choice of analysis time in survival analysis where the hazard is associated with age***From:*roland andersson <rolandersson@gmail.com>

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