Survival Rates and Mortality Rates aren't interchangeable

12/22/2013 06:35:00 PM
Aaron Carroll has an excelent video up explaining the difference between survival rates and mortality rates. It's a very important point, and I frequently see people get it wrong. Especially politicians, who tend to think it's ok to use whichever one makes their point better...but as Aaron Carroll explains, these aren't measures of the same thing, and using the survival rates to compare healthcare systems is just invalid, plain and simple.

The reason is that survival rates depend on the timing on the diagnosis, while mortality rates do not. Regardless of what effect it actually has on people's lifespans, an earlier diagnosis will always increase the 5-year survival rate, because the survival rate is calculated as the number of people alive a certain number of years from the diagnosis. Unless the course of treatment is so harmful that it kills people a lot sooner, earlier diagnosis will result in a higher survival rate. Let me reiterate, when it comes to survival rates, higher is not necessarily better at all.

If you want to know whether a course of treatment is actually better, you have to use the mortality rate. The mortality rate is just the percentage of the population that dies from a given condition in a certain period of time. If your treatment keeps people alive longer, then the mortality rate will fall. Otherwise, the mortality rate won't fall.

So why bother with the survival rate at all? As Carroll says, the survival rate is what you want to know if you have actually been diagnosed with a disease. For an individual, the mortality rate doesn't really tell you much--just the percentage of the total population that will die from the disease in each time period. It tells you how effective treatments are, but you can't figure out how long you have to live from that. The survival rate, by contrast, does tell you how long you can expect to live, it just doesn't tell you how effective the treatments are.

This is pretty simple. Mortality rate tells you about effectiveness of treatment across a population. Survival rate tells you what to expect once you've been diagnosed. Don't confuse the one for the other.