Re: st: Survival analysis multiple events

[Ben Andagalu <ben.andagalu@gmail.com>]

Thanks a lot
Ben Andagalu MD

On Wed, May 26, 2010 at 3:08 PM, Steve Samuels <sjsamuels@gmail.com> wrote:
> There is no need to do _anything_ about the 28 day period if you use
> the "conditional risk set model (time from the previous event)" of
> section 3.2.4.  Just use the dates that you now have.  You will need
> to add a time0 = 0 to start each interval and a stratum variable to
> indicate the number of prior failures.
>
> You could just as well use time0 = 28.  The only difference would be a
> shift in the baseline survival curves S(t).  In the first formulation,
> S(t) = 1 for t<=28 provided there are no failures under treatment.
> The Cox model results will not change at all.   If you don't
> understand why, consult one of the survival references.
>
> The measurement error in using dates of visits for the dates of events
> should average  +3.5 days or so.  If your period of followup for each
> failure is long in comparison, you probably won't introduce much bias
> into the Cox parameters.  To get more accurate baseline curves, apply
> the average correction (-3 or -4) to the date of visit.  This will
> center the measurement error closer to zero.
>
> Steve
>
> On Wed, May 26, 2010 at 10:23 AM, Ben Andagalu <ben.andagalu@gmail.com> wrote:
>> My intention was to use the Cox regression model taking into account
>> potential clustering of events at the individual level.
>> The "not at risk" is due to the treatment given after experiencing the
>> event. the failures were detected at weekly follow-ups and unplanned
>> visits
>> I had actually read section 3.2.4 at
>> www.stata.com/support/faqs/stat/stmfail.html and other literature
>> prior to posting, and my major problem remains to be how to add those
>> 28 days. I started using Stata not so long ago, so pardon my
>> ignorance.
>> Thanks
>> Ben Andagalu, MD
>>
>> On Wed, May 26, 2010 at 1:31 PM, Steve Samuels <sjsamuels@gmail.com> wrote:
>>>
>>> The 28 day problem aside,  what analyses did you have in mind?
>>> Someone can be "not at risk" for different reasons, and the specific
>>> setting could affect the choice of analysis.
>>>
>>> Have you read www.stata.com/support/faqs/stat/stmfail.html ? Your data
>>> are almost set up for the "conditional risk set model (time from the
>>> previous event)" , discussed in section 3.2.4. This model resets the
>>> clock after each failure. You can do Cox regression, and  baseline
>>> survival curves computed for the time period after each failure will
>>> remain at 1.0  for at least 28 days.
>>>
>>> This model would not be appropriate if you believe that risk of
>>> failure is much more strongly related to time since enrollment than to
>>> time from previous failure.  If you need another model,   -snapspan-
>>> will convert your data to a format that -stset- can use, You might
>>> consider withdrawing people from observation in the 28 day period
>>> after each failure. You do this by adding 28 days to the start date of
>>> the interval.  The resulting survival curves and inference will refer
>>> to people who are conceptually "at risk" for the entire time after
>>> enrollment, even if this was not true of the people under study. The
>>> section in the Stata Survival manual on -stset- has many examples
>>>
>>> There could be another issue.  Is the "date of follow-up" visit the
>>> date a failure occurred or did the failure occur at an unknown date
>>> prior to the visit?   If the latter, you  have either grouped data, if
>>> the follow-up visits are at fixed times from enrollment, or
>>> interval-censored data, if the follow-up intervals are not the same
>>> for all people.
>>>
>>>
>>> Good references are:
>>> 1) Stephen Jenkins's text  Survival Analysis using Stata:
>>> http://www.iser.essex.ac.uk/survival-analysis
>>> 2) An Introduction To Survival Analysis Using Stata (Paperback)
>>>  by  Mario A. Cleves  William W. Gould , and  Roberto G. Gutierrez
>>>
>>> Steve
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Tue, May 25, 2010 at 3:23 PM,  <ben.andagalu@gmail.com> wrote:
>>> > How do i go about setting up dataset to analyse multiple failures per individual with a 28 day 'not at risk' period after each failure? individuals were followed up weekly after randomization - The variables i have are: date of randomization, date of follow-up visit, visit id, failure event , patient id, and the predictor variables. The data is in the 'long' format. Thanks. Ben Andagalu.
>>> >
>>> >
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>>> >
>>>
>>>
>>>
>>> --
>>> Steven Samuels
>>> sjsamuels@gmail.com
>>> 18 Cantine's Island
>>> Saugerties NY 12477
>>> USA
>>> Voice: 845-246-0774
>>> Fax:    206-202-4783
>>>
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>>
>> *
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>
>
>
> --
> Steven Samuels
> sjsamuels@gmail.com
> 18 Cantine's Island
> Saugerties NY 12477
> USA
> Voice: 845-246-0774
> Fax:    206-202-4783
>
> *
> *   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/
>

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