What is survival analysis? Is it just analysis of survival data? Well, like many plausible sounding statements, that one is wrong. No, it's not only used to analyze survival data. That's why it's also called time to event analysis, because the event of interest isn't always death. Survival analysis is not just one method, but a family of methods. In this course, we'll go through the two most common ones. So I'm now going to explain what kinds of event can be analyzed this way, and then how this type of analysis differs from logistic regression, which also analyses binary events, those that either happen or they don't. So first, what kinds of events can be analyzed using survival analysis methods. By events, I mean patient outcome, or what happened to the patient during or by the end of the study. I'm going to use events and outcome interchangeably. The classic outcome is death, which can be from any cause or it can be from a specific cause, say cardiovascular disease or cancer. But not just death there, you can also use it for disease relapse such as tumor recurrence, hospital readmission, joint prostheses failure, and many other things. With these nonfatal outcomes, you do need to be a bit careful in your analysis and interpretation, when it comes to people who die before having any of those outcomes. I'll explain more on that later in the course. In logistic regression, you're interested merely in whether the outcome happens, it does not matter when it happened. In contrast, in survival analysis, we're interested not just in whether the patient had the outcome of interest, but also how long it took them to get that outcome, that is, the time to event. Let's say you want to know the effects of lifestyle, so things like diet and exercise, on mortality. You first recruit a set of patients who have not yet experienced outcome. That's pretty easy. As in this case, you just want people who are alive. Then, you measure their lifestyle in some way, for instance by a questionnaire and maybe some lab tests, and you follow them up for several years and wait for them to die. Put that way it sounds callous, I know, but that's how the study works. You then assess the relationship between things like diet and exercise and the risk of death. You can rarely wait for everyone in the study to die. So at some point the study ends, say after ten years, and analysis begins. Now it clearly matters a lot whether people died after a month or after 10 years, huge difference. There's another important point here regarding a difference between the logistic regression and survival analysis. So in that lifestyle study, following people up for 10 years, it's fairly common for some people to be what's called lost to follow-up, before the end of the study. They may move away, or they may decide that they've had enough of being in your study and dropout, especially if you do medical tests on them and ask them to fill out lengthy questionnaires all the time. If they dropout from the study and die, you might not be able to track down a death certificate or their entry in a death registry. So you don't know if they're still alive. That's a problem. For non death outcomes, this can even be harder to ascertain. This is a problem for logistic regression which relies on knowing the facts of death for everyone, but it's not always a problem for survival analysis. So, in summary, survival analysis can be used to explore the relation between patient factors of interest and the time to any binary event. Logistic regression is only interested in the facts of that binary event. This event can be death but it can also be disease recurrence or host of other things. So Survival Analysis is used a great deal in Public Health. So it's an important addition into your statistical toolbox.