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  • #61
    Hello again Mr. Schechter.
    I apologize for spamming with several posts in a row, but i was wondering if you could help me understand how to interpret the stata-outputs from cox-regression with time-varying covarites.

    Are the estimates that i get under "Main" still the hazard ratios that i would report in a scientific article?

    And are the estimates under "tvc" the rate at which the hazard ratio changes over time (like you wrote previously)?
    Last edited by Jonas Kristensen; 08 Apr 2019, 10:19.

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    • #62
      Yes, both are correct.

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      • #63
        Great! Thank you.
        So how would one report the tvc estimates in an article?
        Most of the studies i have included, use cox-regression, but not one has used time varying covariates? I have read and searched alot on this topic, but it seems that there is not much to find.

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        • #64
          Well, if the variables involved are included only to adjust for their effects, I wouldn't spend much effort on this. I would just report all the coefficients in a table similar to the -stcox- output itself and perhaps put an asterisk noting that these particular variables could not be fit to a proportional hazards model and that these coefficients therefore represent the rate of change of the hazard ratio over time.

          If you have a variable that failed proportional hazards and is a key variable in the model, then you need to explain more. You should, in the text, report that the variable was not amenable to proportional hazards modeling, and indicate that a time-varying hazard with the rate of change indicated by the coefficient was observed.

          You are to be congratulated for taking this level of care in your analysis and reporting. In the medical literature, Cox regression is used with abandon. The PH assumption is typically not even explored or tested and bogus hazard ratios are commonly reported as a result.

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          • #65
            Thank you! It wouldn't feel right to do otherwise

            That was very helpful information! Would it also be necessary to be very explicit in the results section, in terms of how the HR is to be interpreted or would it be fine to report the HR, desribe the association and then refer to the tabel for time varying estimates?

            I have already written most of the article and i am editing it now to fit the results from the analysis with time varying covariates. So far i have written the following in the methods section in regards to TVC: "The proportional hazards assumption was not met for the use of cox proportional regression models, in terms of the variables of interest; stroke severity and civil status. A cox regression model with time varying covariates was therefore applied to estimate the effect of stroke severity and civil status on the cause specific hazard of fall-related fractures. Coefficients representing the rate of change of the hazard ratios were calculated and reported for all variables that could not be fit to cox proportional hazards model."

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            • #66
              What you wrote sounds good to me. Since this is not commonly done, and since my involvement in this thread may be providing me with a level of understanding a typical reader of your manuscript won't have, I suggest that you show your draft manuscript to somebody who has had no involvement with the project up to this point, but who knows enough about the general topic to understand it, and ask them to read it and give you comments. Ultimately, the purpose of writing is to share information with the reader. The success of that endeavor can only really be judged by a reader.

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              • #67
                Dear Mr. Schecter. Thank you for your last reply and advice.
                In my results i have included only the first fracture as an outcome and therefore censored multiple fractures for the same patient. If i wanted to include multiple fractures pr. patient, so the patient would not be censored at the first event of fracture, how should i change the outcome command in the command list below?

                Code:
                sort person_id stroke_date
                by person_id (stroke_date): gen second_stroke_date = stroke_date[2]
                by person_id (stroke_date): gen start_follow_date = stroke_date[1]
                by person_id (admission_date), sort: egen first_post_stroke_fx_date = ///
                min(cond(has_fracture_now & admission_date > start_follow_date, admission_date, .))
                by person_id: egen diagnosiscode = min(cond(admissiondate == first_post_stroke_fx_date, diag, .))
                by person_id (admission_date): gen end_follow_date = min(td(31dec2017), dødsdato, second_stroke_date)
                
                gen dead = 1
                replace dead = 0 if missing(death_date)
                by person_id: egen died = max(dead)
                by person_id: keep if _n == 1
                
                gen outcome = !missing(first_post_stroke_fx_date)
                gen endpoint = min(first_post_stroke_fx_date, end_follow_date)
                stset endpoint, failure(outcome = 1) scale(365.25) origin(start_follow_date)

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