- Genuine semiparametric modeling
- Left-censoring, right-censoring, interval-censoring
- Current-status and general interval-censored data
- Stratified estimation
- Two estimators for baseline hazard
- Two estimators for standard errors
- Graphs of survivor, cumulative hazard, and hazard functions
- Residual diagnostics
- Graphical checks of proportional-hazards assumption

What is the exact time of cancer recurrence? Or time of COVID-19 infection? We don't really know exactly. All we know is that the cancer recurred sometime between two follow-up examinations and that the infection occurred sometime before the onset of symptoms or a positive COVID test. These are examples of interval-censored and left-censored time-to-event data. By time-to-event or event-time data, we also mean failure-time data, duration data, and so on.

Interval-censored event-time data arise in many areas, including medical, epidemiological, economic, financial, and sociological studies. Ignoring interval-censoring will often lead to biased estimates.

A semiparametric Cox proportional hazards regression model is used routinely to analyze uncensored and right-censored event-time data. In Stata 17, you can use the new estimation command **stintcox** to fit the Cox model to interval-censored event-time data.

Just as with right-censored data, a Cox model is appealing for interval-censored data because it uses semiparametric estimation and thus does not require any parametric assumptions about the baseline hazard function. Also for low-event rates, the exponentiated regression parameters approximate the log relative-risks.

Semiparametric estimation of interval-censored event-time data is challenging because none of the event times are observed exactly. Thus, "semiparametric" modeling of these data often resorted to using spline methods or piecewise-exponential models for the baseline hazard function. Genuine semiparametric modeling of interval-censored event-time data was not available until recent methodological advances, which are implemented in the **stintcox** command.

- Estimation
- Graphing survivor functions
- Stratification
- Checking proportional-hazards assumption
- Additional resources

We use data from Zeng, Mao, and Lin (2016), who study time to HIV infection in a cohort study of injecting drug users in Thailand. The dataset contains 1,124 subjects. Those subjects initially tested negative for the HIV virus. They were then followed and assessed for HIV-1 seropositivity through blood tests approximately every four months. Because the subjects were tested periodically, the exact time of HIV infection was not observed, but it was known to fall in intervals between blood tests with the lower and upper endpoints recorded in variables **ltime** and **rtime**.

We want to identify the factors that influence time to HIV infection. The factors we are interested in are age at recruitment (**age**), sex (**male**), history of needle sharing (**needle**), history of drug injection (**inject**), and whether a subject has been in jail at the time of recruitment (**jail**).

.use https://www.stata-press.com/data/r17/idu(Bangkok Tenofovir Study (BTS)) .describeContains data from https://www.stata-press.com/data/r17/idu.dta Observations: 1,124 Modified Bangkok IDU Preparatory Study Variables: 8 15 Dec 2020 13:34 (_dta has notes)

Variable Storage Display Value | ||||||

name type format label Variable label | ||||||

age byte %8.0g Age (in years) | ||||||

male byte %8.0g yesno Male | ||||||

needle byte %8.0g yesno Shared needles | ||||||

jail byte %8.0g yesno Imprisoned | ||||||

inject byte %8.0g yesno Injected drugs before recruitment | ||||||

ltime double %10.0g Last time seronegative for HIV-1 | ||||||

rtime double %10.0g First time seropositive for HIV-1 | ||||||

age_mean double %10.0g Centered age (in years) | ||||||

We fit a Cox proportional hazards model in which time to HIV infection depends on the above factors of interest. To make the interpretation of the baseline hazard function more reasonable, we will use the centered age variable, **age_mean**.

.stintcox age_mean i.male i.needle i.inject i.jail, interval(ltime rtime)note: using adaptive step size to compute derivatives. Performing EM optimization (showing every 100 iterations): Iteration 0: log likelihood = -1086.2564 Iteration 100: log likelihood = -597.65634 Iteration 200: log likelihood = -597.57555 Iteration 295: log likelihood = -597.56443 Computing standard errors: ............................ done Interval-censored Cox regression Number of obs = 1,124 Baseline hazard: Reduced intervals Uncensored = 0 Left-censored = 41 Right-censored = 991 Interval-cens. = 92 Wald chi2(5) = 17.10 Log likelihood = -597.56443 Prob > chi2 = 0.0043

OPG | ||||||

Haz. ratio std. err. z P>|z| [95% conf. interval] | ||||||

age_mean | .9684341 .0126552 -2.45 0.014 .9439452 .9935582 | |||||

male | ||||||

Yes | .6846949 .1855907 -1.40 0.162 .4025073 1.164717 | |||||

needle | ||||||

Yes | 1.275912 .2279038 1.36 0.173 .8990401 1.810768 | |||||

inject | ||||||

Yes | 1.250154 .2414221 1.16 0.248 .8562184 1.825334 | |||||

jail | ||||||

Yes | 1.567244 .3473972 2.03 0.043 1.014982 2.419998 | |||||

We find that age at recruitment is associated with lower risk of HIV infection and being in jail at enrollment is associated with higher risk of HIV infection. Other factors are not statistically significant.

We can graph the baseline survival curve by using **stcurve** with all covariates set to 0.

.stcurve, survival at((zero) _all)

We can also graph the survivor functions with covariates set to any value. Suppose we want to compare the survivor functions of an average subject in the female and male groups. We type

.stcurve, survival at(male=(0 1))

If we assume that the baseline hazard function for **female** is different from that for **male**, we can fit a stratified Cox proportional hazards model by using the **strata()** option:

.stintcox age_mean i.needle i.inject i.jail, interval(ltime rtime) strata(male)note: using adaptive step size to compute derivatives. Performing EM optimization (showing every 100 iterations): Iteration 0: log likelihood = -1087.0536 Iteration 100: log likelihood = -585.59848 Iteration 200: log likelihood = -585.53143 Iteration 282: log likelihood = -585.5222 Computing standard errors: ........................... done Stratified interval-censored Cox regression Baseline hazard: Reduced intervals Strata variable: male Number of obs = 1,124 Uncensored = 0 Left-censored = 41 Right-censored = 991 Interval-cens. = 92 Wald chi2(4) = 14.84 Log likelihood = -585.5222 Prob > chi2 = 0.0051

OPG | ||||||

Haz. ratio std. err. z P>|z| [95% conf. interval] | ||||||

age_mean | .9682508 .0126326 -2.47 0.013 .9438052 .9933295 | |||||

male | ||||||

Yes | 1.276222 .2270302 1.37 0.170 .9005422 1.808625 | |||||

needle | ||||||

Yes | 1.245357 .2393768 1.14 0.254 .8544367 1.815131 | |||||

inject | ||||||

Yes | 1.245357 .2393768 1.14 0.254 .8544367 1.815131 | |||||

jail | ||||||

Yes | 1.57314 .3490687 2.04 0.041 1.018337 2.430205 | |||||

Our conclusion here is similar to the case without stratification.

It is important to evaluate the validity of the underlying assumption for the Cox proportional hazards model, which is that the hazard ratio is constant over time. Stata 17 provides two new graphical commands to assess the proportional-hazards assumption.

For a single categorical covariate in a Cox model, you can use **stintphplot**, which plots -ln{-ln(survival)} curves for each category versus ln(analysis time). If the plots are parallel, the proportional-hazards assumption has not been violated. Alternatively, you can use **stintcoxnp** to plot the nonparametric maximum-likelihood estimation survival curve versus the Cox predicted survival curve for each category. When the two curves are close together, the proportional-hazards assumption is valid.

When a Cox model contains multiple covariates, as mentioned in the above example, only **stintphplot** is appropriate for testing the proportional-hazards assumption. In that case, we need to use the **adjustfor()** option.

For example, to check the proportional-hazards assumption for **inject**, we include all covariates except **inject** in the **adjustfor()** option:

.stintphplot, interval(ltime rtime) by(inject) adjustfor(age_mean i.male i.needle i.jail)

A separate Cox model, which contains all covariates from the **adjustfor()** option, is fit for each level of **inject**. And the two plots are almost parallel, which indicates that the proportional-hazards assumption has not been violated for the categorical variable **inject**.

Zeng, D., L. Mao, and D. Lin. 2016. Maximum likelihood estimation for semiparametric transformation models with interval-censored data. *Biometrika* 103: 253–271.

Learn more in **[ST] stintcox**.

Learn more about parametric interval-censored event-time models.