Time-dependent survival estimation in post–polycythemia vera myelofibrosis. Survival curves estimated from the Cox proportional hazard regression with time-dependent covariates. According to the model, each patient is initially assigned to a risk group and followed in that group as long as no changes in risk factors take place. The patient is reassigned to another risk group whenever further risk factors are acquired. So, each patient may contribute with some observation time to the estimate of survival in different risk groups. Therefore, the upper curve includes patients who did not acquire any risk factors during the whole follow-up (ie, hemoglobin level ≥ 100 g/L [10 g/dL], platelet count ≥ 100 × 10⁹/L, or white blood cell count < 30 × 10⁹/L). The other curves include patients who acquired 1, 2, or 3 factors during follow-up.