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I have a Cox regression problem where the dependent variable is time to some clinical event. One of my covariates is an intervention that may or may not occur during follow-up, and if it occurs, the time between baseline and the start of the intervention differs from patient to patient. Now my questions are as follows:
- Since Cox regression in SPSS does not support data in the 'long' format (at least so I understood), how can I analyze data where the time varying covariate changes more often than once (e. g., an intervention that has a start and an end, both of which may affect the hazard, or a series of successive interventions)?
- As I have written above, most of my patients receive the intervention coded in a time varying covariate, but not all. My understanding of the output of Cox regression with time varying covariates is that the hazard ratio for this covariate applies to the time until the intervention occurs. How do I interpret the coefficient in case of patients in whom the intervention does not occur at all?
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