Screening screening - Numbers Rule Your World
One of the metrics used to measure health outcomes is the five-year survival rate… what proportion of patients are still alive five years after they are diagnosed with breast cancer? Said differently, we are interested in the length of time between the time of diagnosis and the time of death. There are two ways to improve this metric, by delaying the time of death, or by moving earlier the time of diagnosis. The former is difficult to achieve, and requires genuine progress in medication. The latter is very easy! If we screen younger people, we will move the time of diagnosis earlier. Without any medical progress, we will make the five-year survival rate fall. This phenomenon is known as lead-time bias.
Expanding screening to younger and less vulnerable populations has another effect, which I discussed in Chapter 4 of Numbers Rule Your World. The marginal person who gets diagnosed will be less ill than the average person - Orenstein tells us that the form of disease most commonly diagnosed in younger women (DCIS) ought not even to be called “cancer”. Add less ill people to the population of ill people will automatically improve the survival rate, again without any medical progress. This phenomenon is known as length bias.